Diurnal variations of light interception and photosynthesis in relation to canopy structure of cotton (Gossypium hirsutum L.) were studied. The three-dimensional (3D) electromagnetic digitizing system was used for measuring plant geometrical structure under field-growing condition. The 3D version of RIRI (Radiation Interception in Row Intercropping) model based on the turbid medium analogy was used to simulate light-interception distribution and net CO₂ assimilation rate of the canopy during 3 stages of development, i.e. at the leaf area index (LAI) of 0.7, 1.2 and 3.1. The net assimilation rates simulated by the model were compared with canopy photosynthesis measurement.