Based on the results of gas flow field, droplet size distribution and droplet initial flow rate distribution on aф300mm rotating stream tray (RST), a three dimensional droplet projection trajectory model and a mass transfer efficiency model were developed. The calculation of the models was carried out by using a finite-difference procedure1 The predicted results of the plate efficiency agreed quite well with the experimental data measured with absorbing NH3 or SO2 on aф300mm RST and the operation data from a commercial ф3600mm RST column1 Meanwhile, the effect of gas-liquid system and RST's structure on plate efficiency was discussed.