The mode coupling characteristic in the asymmetrical coupler is studied theoretically and experimntally. The cirtical fiber mismatch criterion of r2/r1=0.62～0.63 is given. It is shown that when the fiber mismatch exceeds the limit, higher order mode coupling occurs and the splitting ratio decreases. By controlling the fiber mismatch properly, the fiber null coupler with splitting ratio of 5×10 3～10 5 and loss of 0.1～0.2 dB is made.