The filtration performance of square fiber was studied by using a Brownian dynamics simulation technique, which allowed for simultaneous consideration of all the important mechanisms of particle capture including Brownian diffusion, interception and inertial impaction. The flow around square fiber used for evaluating the particle trajectories was solved numerically. The effects of orientation angle θ, the fiber volume fraction C and the filtration velocity u∞ on capture efficiency, quality factor as well as the distributions of deposited particle were discussed and analyzed. The results showed that the distributions of deposited particle over a single square fiber were characterized by localized regional deposition for both diffusional deposition of small particles and inertial capture of large particles, and were significantly influenced by capture mechanisms and orientation angles. According to the calculated results for the filtration pressure drop and capture efficiency, the quality factor of square fibers with various orientation angles were then evaluated, and the results indicated that in the case of high fiber volume fraction, the square fiber showed higher comprehensive filtration performance than the cylindrical fiber even though its filtration pressure drop was much higher. However, in the case of low fiber volume fraction, the square fiber showed lower comprehensive filtration performance than the cylindrical fiber.