An approxima te solution has been presented for the single-input nonlinear optical direction al coupler under the most commonly used condition:operating near the critical p ower.By introducing the approximation in previous step,a simplified integral equ ation has been obtained.It can be integrated as simple functions instead of elli ptic functions.Numerical examples of our results have been given and compared wi th the exact solution and the approximate solution derived by A.T.Pham et al.Our results are more accurate than that of A.T.Pham et al.