Supplemental lighting is a key technology in vegetable nursery greenhouses to improve transplant growth and quality. Light emitting diodes (LEDs) are a newest addition to current supplemental greenhouse lighting technology. However, LED lighting technology must be evaluated in terms of economic feasibility and plant responses. At the University of Arizona, a two-phase study was conducted to evaluate supplemental LED lighting technology for vegetable transplant production in greenhouse. Tomato (Solanum lycopersicum 'Komeett') and cucumber (Cucumis sativus 'Cumlaude') were examined. The first phase study consisted on evaluating blue light supplemental lighting requirement under daily solar light integrals of 6-9 mol m-2 d-1 created by shade screens deployed in the greenhouse. The LEDs used for supplemental lighting were a mix of red and blue providing an average of 55 μmol m-2 s-1 PPF for 18 hours (2 am - 8 pm) or 3.5 mol m-2 d-1 over the plant canopy surface. There were three treatments consisted of varied % blue photon flux: 0% blue (0 μmol m-2 s-1), 4% blue (2.2 μmol m-2 s-1), 16% blue (8.8 μmol m-2 s -1) and a control with no supplemental lighting. Blue and red LEDs had peak wavelengths of 455 nm and 661 nm respectively. Plant growth of the young seedlings were largely improved by supplemental LED lighting in both species compared to the control (no supplemental lighting). For example, dry mass was increased by 39% for tomato and 47% for cucumber. For tomato no significant differences in dry mass were observed among different % blue treatments. However, cucumber plants showed significant reduction in dry mass, when the % blue photon flux was increased. The second phase study consisted of a side-by-side comparison of 100% red LED (632 nm peak wavelength) and a conventional high pressure sodium (HPS) lamp at the same light intensity and photoperiod used in the first phase study. Preliminary results showed that plant dry mass under red LED was non-significantly different from that under HPS for tomato but was 25% lower under red LED than HPS for cucumber. Based on the results from the phase one study, it is clear that for 'Komeett' tomato and 'Cumlaude' cucumber, 100% red supplemental LED is sufficient to improve growth of the seedlings. From the phase two preliminary results, it seems that growth of tomato seedlings under 100% red LED was comparable to that under HPS light but the growth of cucumber seedlings was greater under the HPS than the 100% red LED lighting. From this study it is evident that plant species respond differently to supplemental lighting and in order to improve growth and morphology in greenhouse crops the selection of supplemental lighting should be species specific.