Every person entering Ukraine must have a valid Visa issued by the Ukrainian Embassy or the Consulate office in the country of the traveler's residency or origin. A Ukrainian visa for entering the country is required for citizens of most countries.
No visas are required for citizens of Romania, Hungary, Poland, Mongolia, Serbia and Montenegro, and some countries of the Commonwealth of Independent States.
Note that the Ukrainian Government doesn't issue visas at the time of entry into Ukraine. One should obtain a visa in advance, before traveling. If you are transiting the country, without staying there, a visa is also required.
There are several types of visas one can apply for. Visa holds information on type, term of validity and usually reflects your itinerary in Ukraine, which is based on application and visa supporting documents you provide the Consulate with.
A traveler to Ukraine must have a valid USA/Canadian Passport, the validity of which does not expire at least 30 days after scheduled return from Ukraine. Stateless or USA permanent residents must have a valid Re-entry permit (white passport) and alien card.
Find more infromation on Visa to Ukraine web site.