The optical performance of Parabolic Trough Collectors (PTCs) for various tracking modes in sub-tropical area (for latitude of 25.02°) were investigated with Hottel model, incidence angle and daily irradiation during equinoxes and solstices, as well as annual incident irradiation were all compared. Results show that, the mode of tracking affects the amount of incident radiation falling on the collector surface, for four tracking modes, PTCs obtain much more irradiation in summer and much less in winter, and the optimum period for solar thermal application is between March and October. Besides, for two axes rotating tracking, polar E-W tracking, horizontal E-W tracking and horizontal N-S tracking, the relative proportion of yearly irradiation yield are 100%, 95.93%, 91.32% and 73.67% respectively. The selection of tracking mode for PTCs should not only take into consideration of temperature delivered, heat demand level, but also all the costs for manufacture, the auxiliary energy required for tracking drive, and other overheads.