A series of transparent ABS(T-ABS) resins were prepared by emulsion in situ suspension polymerization. The influences of the particle size and the content of rubber particles on the transparency of T-ABS resins were studied by varying the size and content of rubber particles in a single model system( rubber particles with a uniform size). The optical properties of T-ABS resins were investigated in a mixed system of SBR/PB particles and a bi-modal particle system(rubber particles with two different sizes, 70 and 400 nm in diameter) of SBR particles. It was found that when the size of the smaller particles (70 nm) in the mixed system of SBR/PB particles was in the range of 50-100 nm in diameter, the T-ABS resins showed a better transparency. These results provide a flexible and practical process for the preparation of T-ABS resins with good optical and mechanical properties.