In order to investigate the effect of light wavelength on growth and physiology of lettuce plants, Romaine lettuce and sunny leaf lettuce were chosen as targets for the study. Using single color LED lights (white, red, green, blue), leaf absorption of light, photosynthetic rate, stomatal conductance and transpiration rate of romaine lettuce leaves were examined. The results showed low correlation of light absorption percentage and stomatal conductance to photosynthetic rate of romaine leaf. However, transpiration rate and photosynthetic rate have high correlation, suggesting a relationship between these two processes. Sunny lettuce plants were cultivated on rockwool using hydroponic method under different light source including fluorescent lamb and LED of different colors (white, red, green, blue and rainbow). Plants illuminated by fluorescent lamp gave the highest dry weight, while rainbow LED provided plants with the lowest fresh and dry weight. Plants under white, red, green or blue LED do not differ significantly about fresh and dry weight. These results suggest that combination between different types of LED is required to achieve the best yield of lettuce plants.