In order to obtain basic data on the effective application of artificiallights and colored transparent shelters to plant cultivation, effects of composite lights with a dominant energy in the blue (B), green (G), red (R) and blue and red (BR) on photosynthesis of the following crop plants were investigated: tomato (Lycopersicum esculentum Mill.), sweet pepper (Capsicum annuum L.), strawberry (Fragaria grandiflora Ehrh.), cucumber (Cucumis sativus L.) and squash (Cucurbita moschata Duch.) as vegetable species; and peach (Prunus persica Sieb. et Zucc.), grape (Vitis aestivalis Micahux) and satsuma mandarin (Citrus unshiu Mark.) as fruit tree species. At an irradiance 50 W m-2 in the waveband 400-700 nm, which gives a rate of about 1/3 of the maximum photosynthesis, the photosynthetic rate relative to white (W) light was generally higher in R, slightly lower in BR and B and clearly lower in G for both species of vegetables and fruit trees. This difference among the light qualities was negligible at irradiances close to the light-saturation point, but extended with decreasing the irradiance. Photosynthetic efficiency, which was expressed in terms of photosynthetic rate at the original slope of light-response curve, was raised with increasing the energy in the red region when the efficiency was shown for unit energy incident (400-700 nm), while the efficiency for unit quanta incident (400-700 nm) was not much different among the light qualities. A possibility of the prcsence of enhancement effect for photosynthesis was found in most of the composite lights, assuming that no enhancement effect is present in G light. The enhanced part, however, did not exceed more than 10 percent even in W light in the average of the species. From the data of the present study and the literature, it was concluded that red light energy is the most important to photosynthesis and that the relative effectiveness of composite lights on photosynthesis can be roughly estimated by means of the sum of the products of the averagc action spectrum by the energy distribution of each light, or by the quantum flux density incident in the waveband 400-700 nm of each light.