It has been reported that Arabidopsis phytochrome (phy) A and phyB are crucial photoreceptors that display synergistic and antagonistic action during seedling de-etiolation in multiple light signaling pathways. However, the functional relationship between phyA and phyB is not fully understood under different kinds of light and in response to different intensities of such light. In this work, we compared hypocotyl elongation of the phyA-211 phyB-9 double mutant with the wild type, the phyA-211 and phyB-9 single mutants under different intensities of far-red (FR), red (R), blue (B) and white (W) light. We confirmed that phyA and phyB synergistically promote seedling de-etiolation in B-, B plus R-, W- and high R-light conditions. The correlation of endogenous ELONGATED HYPOCOTYL 5 (HY5) protein levels with the trend of hypocotyl elongation of all lines indicate that both phyA and phyB promote seedling photomorphogenesis in a synergistic manner in high-irradiance white light. Gene expression analyses of RBCS members and HY5 suggest that phyB and phyA act antagonistically on seedling development under FR light.