The magnetic field at the center of a long circular coil carrying current will be :

The field at the center of a long circular coil carrying current will be parallel straight lines.  

A circular coil that has current flowing through it behaves like a magnet when one of its faces is the North pole and the other the South pole of a magnet.

Now, if you draw the field lines of the object like a bar magnet, you'll discover that they all loop around the wire. They will enter at the south pole face of the coil and exit from the north pole. As a result, they will appear straight and parallel in the middle.