In this study, a vortical stator assembly (VSA) was developed to improve the rotor performance, surrounding a drag-type, vertical-axis wind turbine (VAWT). A rotor with six half-tube blades was employed. The VSA consisted of a number of guide vanes, which were to guide the air tangentially into the VSA. The design was to generate a vortical flow inside the VSA, to reduce the negative torque of the returning rotor blade, and to increase the air speed into the rotor. Wind tunnel model tests were conducted to investigate how the VSAs affected rotor performance. Four VSAs were developed in this study, employing 4, 6, 8 and 12 guide vanes that were 24cm in width, and one 6-guide-vane VSA employing guide vane widths of 12, 18, 24 and 30cm. Results indicated that VSAs can substantially augment the rotor performance, depending on the number and length of guide vanes. The 6-guide-vane VSA produced the largest effect on the rotor performance, and the optimal VSA diameter was approximately 1.82 times the rotor diameter. At an air speed of 6m/s, the optimal VSA helps to increase the free-running rotational speed by 318%, the torque output by 200%, and the maximal power output by 910%.