The nonlinear switching characteristics of fused fiber directional couplers were studied experimentally. By using femtosecond laser pulses with pulse width of 100 fs and wavelength of about 1550 nm from a system of Ti:sapphire laser and optical parametric amplifier (OPA), the nonlinear switching properties of a null coupler and a 100% coupler were measured. The experimental results were coincident with the simulations based on nonlinear propagation equations in fiber by using super-mode theory. Nonlinear loss in fiber was also measured to get the injected power at the coupler. After deducting the nonlinear loss and input efficiency, the nonlinear switching critical peak powers for a 100% and a null fused couplers were calculated to be 9410 and 9440 W, respectively. The nonlinear loss parameter P_(N) in an expression of α_(NL)=αP/P_(N) was obtained to be P_(N)=0.23 W.