In this paper, we design an adaptive sliding mode controller applied on trajectory tracking of robot arms. Consider the uncertainties and external disturbances of a robot arm, we choose sliding mode control (SMC) to be the major controller and propose an adaptive control to tune the switch gain of SMC that would be able to handle unknown disturbances and uncertainties of the system. The adaptive control makes system transient response slow, so we include an exponential law (EL) to make system transient response faster. We not only combine EL with SMC to reduce chattering, but also combine EL with adaptive control to be the variable step-size adaptive control (VSSAC) which step size is tuned by error. Consequently, we prove the stability of system and show excellent performance of this controller by the experiment results.