Based on discussing the coupling characteristics between two curved waveguides, an improved directional coupler is presented, namely, substituting two curved waveguides for the straight waveguides used in the conventional directional coupler. The FD-BPM, based on PML (perfectly matched layer), is applied to analyze the relationship between the insertion loss, power loss and the separation between the waveguides in the coupler region both the conventional and improved one. Configuration parameters of 3-dB couplers are worked out according to two kinds of directional couplers mentioned above. To achieve wavelength-flattened response, an asymmetric structure is also used. The configuration and performance of improved directional coupler has improved obviously, compared with the conventional one.