Abstract: To investigate the relationship between greenhouse tomato(Lycopersicon esculentum Mill.) leaf stomatal resistance and greenhouse microclimate factors and the possibility to use the Penman-Monteith equation to simulate greenhouse tomato transpiration under subtropical climate conditions, experiments with two varieties were carried out in a Venlo type greenhouse in Nanjing (118°E, 32°N) during spring and winter season in 2003～2004. The results showed that the relationship between leaf stomatal resistance r_s and PAR (photosynthetically active radiation) follows the same function (r_s=11205/(5.28+PAR)) for both tomato varieties. The combination of the r_s-PAR function with the P-M equation gives satisfactory estimation of greenhouse tomato transpiration during both spring and winter season. The correlation determination coefficient(R²) and standard error(SE) between the simulated and measured accumulated tomato transpiration based on the 1∶1 line are 0.97 and 5.50 mm, respectively, for spring season and 0.99 and 1.21 mm, respectively, for winter season. The quantitative relationship between tomato leaf stomatal resistance and PAR determined in this study provides a solution for the problem of stomatal resistance estimation when using P-M equation to simulate greenhouse tomato transpiration rate in Yangtze River Delta and enhances the applicability of the P-M equation to greenhouse tomato water management practices.