A major disadvantage of applying sliding mode control (SMC) to dc-dc buck converter is that switching frequency is affected by line and load variations. To maintained constant switching frequency, two different methods incorporated to design a sliding mode controller for dc-dc buck converter is presented in this paper. The first method is called direct SMC, employed as a general sliding mode control (SMC) with hysteresis method to suppress the switching frequency in which output of SMC controller is applied directly to power switch. The second method, Indirect sliding mode control, is based on the modified integral variable structure control (MIVSC), in which the output of the SMC controller applied to the switch through Pulse width modulation (PWM). Two methods are compared by simulation (MATLAB/SIMULINK). The result gives that the indirect SMC is able to suppress the switching frequency and exhibits better load and line disturbance rejection than the direct SMC control method.