Duckweeds with desirable nutritional value and high potential for biomass production have been considered in recent years as promising plants for feeding the world's growing population. Few researches have been conducted on environmental requirements for optimal production of their biomass. In order to study of the effect of supplemental light intensity and spectrum on some growth characteristics of Lemna minor an experiment was conducted in a research greenhouse of Department of Horticultural Sciences, University of Tehran. Experimental treatments included supplemental LED light as white, red-blue spectra in ratios of 1:2 and 2:1 and two light intensity levels of 150 and 250 µmole m-2s-1. Analysis of some indices at the end of cultivation period, showed that the effect of light intensity on growth traits was more effective than light spectrum. Fresh weight, dry weight, relative growth rate, linear growth rate and biomass doubling time were affected by light intensity. The findings of this study confirmed that increasing light intensity improves biomass producing potential of L. minor. Also, the interaction effect of spectrum and light intensity on nitrogen percentage and protein content of biomass was significant. Use of red-blue spectrums compared to white spectrum increased protein content. The results of this research highlighted the importance of the lighting systems in the optimum production of duckweed biomass. Understanding the relationship between light parameters and duckweed physiology will help us to provide appropriate light condition for sustainable biomass production as source of animal feed and food.