I have been working in Unreal and have some projects in it. But now I would like to use the unreal assets in unity3d. Is it legal? If yes, how do I do it?
If they are your own property, have fun. You can export them using Asset Actions → Export.
If they are not your own property, no you cannot use them in unity.
A comment on the answer above. You can use assets purchased on the unreal marketplace inside unity as long as they are not made by Epic Games. Check the FAQ for confirmation. It says:
Can I use these products in other gaming engines, like Source or Unity?
Any Marketplace products that have not been created by Epic Games can be used in other engines. Please note that products purchased from the Marketplace cannot be sold back to the Marketplace after alteration.