The performance of vertical-axis wind turbines (VAWTs) are substantially affected by the phenomenon of dynamic stall which is induced by the variations of angle of attack of rotating blades, especially at low tip-speed ratios (TSRs). Large and sudden torque fluctuations are observed to take place when the dynamic stall vortices, formed near the blade leading-edge, are transported downstream. At low TSRs (lambda(TSR) < 4) and relatively low Reynolds number (Re < 105), dynamic stall occurs periodically during the rotation of turbine blades. This results in a sharp drop in lift coefficient and therefore rotor torque and power output are essentially reduced. The purpose of the present study is to investigate the concepts for improving the power performance of a conventional H-type VAWT model by implementing sinusoidal serrations on the leading-edge of turbine blades to control the dynamic flow separation at low TSRs. A thorough numerical study has been carried out to obtain the detailed flow fields for analysis and visualization. The power output results show that the improved turbine design with the sinusoidal serration profile of the wave amplitude h = 0.025c and the wavelength lambda(s) = 0.33c not only increases the power generation at low TSRs, but also enhances the capability of wind energy extraction at the optimal TSR in comparison to the baseline model. The flow separation is significantly controlled in the azimuth angle ranges from 75 degrees to 160 degrees, where the positive torque generation is also found to be considerably increased in the improved turbine model. Counter-rotating vortex pairs are generated due to the existence of serrations, which suppress the flow separation, especially in the regions near the peak-serration sections. Additionally, the lift coefficients illustrate a delay of occurrence of dynamic stall and a notable improvement of maximum lift in the improved wind turbine model in comparison to the baseline model. The effects of Reynolds number variation also reveal that the improved model would gain more benefits in power generation at low Reynolds number compared with that at high Reynolds number. The simulated results demonstrate that the leading-edge serration strategy could be an effective solution to control the dynamic stall in the operation of VAWTs.