Effect of light intensity and plant sink/source balance on strawberry plant photosynthesis was investigated in this study. Previous research (Inaba, 2007) showed diurnal decline of photosynthetic capacity in strawberry, indicating potential presence of negative photosynthesis feedback. We conducted monthly measurements of leaf net photosynthetic rate (Pn), stomatal conductance, intercellular CO2 concentration (Ci) and transpiration rate in strawberry cultivars 'Albion' and 'Nyoho' in greenhouse, during winter/spring production season. Measurements were taken hourly from 9 am to 4 pm using portable leaf photosynthesis measurement system under 1000 μmol m-2 s-1 photosynthetic photon flux (PPF) and 400 μmol mol-1 CO2. Pn measurements were conducted for 'Speedella' tomato for comparison. Sink load of flowers and fruits was estimated from number of flowers and fruits at the measurement day and the fruit yield from the consecutive 4 weeks. Potential source strength was evaluated based on number of leaves and the daily light integral (DLI, 400-700 nm). We observed seasonal changes in maximum Pn, as well as diurnal change in Pn in both strawberry cultivars. As expected, in tomato, Pn was relatively constant regardless of time of day or month. Analysis showed significant positive correlation between estimated ratios of sink/source and slopes of linear equations applied for diurnal change in Pn. Also, Pn was negatively correlated with Ci but not significantly correlated with vapor pressure deficit in greenhouse, suggesting diurnal decline in Pn was likely due to negative feedback of photosynthesis and not to water stress. Results indicate morning hours as critical time for promoting photosynthesis in strawberry, when negative feedback of photosynthesis is occurring. Understanding diurnal change of plant photosynthetic capacity could lead to develop practices in strawberry production under controlled environment in order to maximize production.