This paper desoribera the behavior of coupling power for fused biconioal linear format 3×3 single mode optical fiber directional coupler using the theory of composited waveguides. The conditions of the cental port input and the outside port input in this coupler are discusses respectively. The expected results are obtained in the case of former, but different results in the case of latter, which is very useful. The analysis for these cases are in a good agreement with experimental results.