Summary: We investigate the robust trajectory tracking controller based on extended disturbance observer (EDO) and sliding mode control algorithm for small-scale unmanned helicopters in the presence of mismatched disturbances. First, a nonlinear mathematical model of unmanned helicopter is presented, to which the approximate linearization technique is applied for partitioning it into two interconnected subsystems: position subsystem and yaw subsystem. Then, by integrating the mismatched disturbances and their derivatives into the new time-varying sliding-mode surface, we develop a novel sliding-mode controller for the helicopter system. Moreover, the stability analysis of the closed-loop system is given. Finally, some comparative simulation results are presented to illustrate the effectiveness and superiority of the proposed control method for unmanned helicopters. This novel sliding-mode controller has two key advantages: insensitivity to mismatched disturbances and suppression of chattering phenomenon.