This study was conducted to investigate the effects of the frame ratio and greenhouse length on the transmissivities of direct and diffuse solar radiation in glasshouse using a computer simulation model developed by Kim and Lee(1997). Transmissivity of diffuse solar radiation slightly decreased as the frame ratio increased. There was no effect of number of spans on the transmissivity of diffuse solar radiation at the same frame ratio. In single or multispan glasshouse, transmissivity of direct solar radiation was 1.5-3.0% higher at the frame ratio of 11.3% than those at the frame ratio of 14.9%. Also the transmissivity of direct solar radiation was 1.5-3.0% lower at the frame ratio of 18.3% than those at the frame ratio of 14.9%. Effect of the increased or decreased frame ratio on the transmissivity of direct solar radiation was similar in I-W or S-N glasshouse. Since the high transmissivity of direct solar radiation exerted a beneficial influence upon the plant growth during winter season, the light and endurable structural members were needed to maximize the transmission of solar radiation in glasshouse. Transmissivity of direct solar radiation in I-W or S-N glasshouse did not vary with the length of 24.5m long or more.