We analytically calculate the equilibrium sequence of the corotating binary stars of incompressible fluid in the first post-Newtonian (PIV) approximation of general relativity. By calculating the total energy and total angular momentum of thr system as a function of the orbital separation, we investigate the innermost stable circular orbit for a corotating binary (we call it ISCCO). It is found that by the first PN effect, the orbital separation of the binary at the ISCCO becomes small with increase of the compactness of each star, and as a result, the orbital angular velocity at the ISCCO increases. These behaviors agree with previous numerical works.