The two are very different ones: 어떤: which 일부: some And thus, 일부 is probably what you are looking for ============================================ Edit: You have a valid point. 어떤 can mean "some" depending on the context. Still, 어떤 just means "some" even in your example while 일부 always means "some of a specified group" 일부 literally means one(일) + part(부) 일부 EU국가들에서는 유로화를 사용하지 않는다. The Euro currency isn't used in some EU countries. Replacing 일부 with 어떤 would be incorrect in the sentence above since the group is clearly specified (EU)

They have similar meaning in certain contexts like your example but I feel that they have a little bit difference on the certainty of the object they describe. Just my thoughts ;)