The German EPP group deputy warned that, "There is a clear danger that as soon as we turn our backs, everything will be as it was before."
Her comments come after a delegation of MEPs visited Ukraine this week.
The visit comes after the EU recently proposed visa-free travel of up to 90 days to Ukrainians. This is seen as a reward to the country for making significant judicial and security reforms.
The proposal needs approval from the European Parliament and EU member states but could come into force late summer.
The delegation of four MEPs concluded their fact finding visit to Ukraine on Thursday. They assessed the financial assistance provided by the EU to support Ukraine's stabilisation and reform process.
The deputies, members of Parliament's budgetary control committee which Grässle chairs, analysed how the aid is being used on the ground, how the country is coping with corruption and whether it affects EU funds.
The delegation, which visited the Lviv region to monitor EU funded projects and trans-border cooperation with the EU, met the central authorities in Kiev. There were also meetings with civil society and anti-corruption watchdogs.
Since the 2013-2014 Maidan revolution, the EU has been providing financial assistance to support Ukraine's stabilisation and reform process.
Some €3.4bn of EU funds go to providing macro-financial assistance, €900m for bilateral development assistance, €500m on regional assistance, €50m for stability and peace and €60m is spent on humanitarian aid.
The EU aid covers the fight against corruption, judicial and public sector reform as well as economic reform and private sector development.
In her assessment of the visit, her first to Ukraine, Grässle told this website, "One of the main aims was to develop an understanding of the country.
"For me, the main finding of the visit is that we need to keep a constant eye on budget support and macro-financial assistance, which both involve considerable amounts of money.
"I cannot help feeling that the sustainability of reforms are in danger and that reformers may only do their work because we Europeans attach importance to it."
Grässle added, "We must not allow interlocutors to report to us just what we want to hear. My impressions about the sustainability of reform efforts in the Ukraine Parliament are also mixed.
"On the one hand, we see a unique setup of new anti-corruption agencies and law enforcement bodies. On the other hand, life is made as hard as possible for these new bodies and agencies, due to missing equipment and cars, inadequate office space and suchlike."
Grässle warned, "There is a clear danger that as soon as we turn our backs, everything will be as it was before."
She added, "Therefore, we need to continue empowering the reformers. Ukraine is a country at war. We see huge reform efforts, in particular as regards the judiciary and public procurement. Some of the results are truly impressive.
"It seems that our money is in good hands whenever it goes directly to beneficiaries. At the same time, we need to devote ourselves to the follow-up of these projects, and most of all macro-financial assistance."
She continued, "Also, budget support has once again shown important weaknesses, in particular regarding accountability.
"Moreover, the effectiveness of the cooperation between the multitudes of donors is a challenge: the World Bank, the IMF, EU member state aid organisations, as well as no less than 17 UN bodies, are all on the ground."
During the four-day visit she said the delegation party had also looked at the work of the European Commission's Ukraine Support Group.
On this, Grässle said, "My impression is that the Commission is very committed and that the EU delegation's presence is having very positive effects."
"But," she cautioned, "We should not underestimate the challenges that still lie ahead."