A new circuit simulator for submicron MOS desings was developed by enhancing SPICE3. The minimum conductance stepping, source stepping and pseudo transient methods are applied to improve the convergence. and SECSPICE uses the variation rate of the node volgage in the timestep algorithm. The modified BSIM model was implemented in SECSPICE for submicron MOS designs. And it gives the powerful user environments such as graphic user environments. As the results of test using real measured device data and circuits used in real production desing, we found it gave more accurage results than BSIM and the execution speed was 1.5~2.8 times faster than SPICE3.