unwrap cube with circle make cube, draw circle and extrude in. while unwrap the cube, it did't unwrap perfectly. its unwrap like what in the picture.
The problem you're having are three N-Gons. N-Gons are faces with more than 4 verts. So you need to change the N-Gons to tris or quads. To triangulate a face, just press SPACE and type triangulate, you will then see the option "triangulate faces".

The first picture shows my unwrap with N-Gons, the second one is with triangles. 




However, the right way of doing it (if you want to maybe use sub-d or something) would be like this. You the only would have to deal with the edges going over the cube.: