De-etiolation during seedling development is antagonistically regulated by blue light (BL) and gibberellins (GAs). The crosstalk between blue light (BL) and GA metabolism and signaling remains unclear. Using the mutant har1 which is specifically hypersensitive to BL in de-etiolation, the involvement possibility of the GA metabolism, GA signaling in the inhibition of mesocotyl elongation of the sorghum (Sorghum bicolor L. var. R111) seeding under BL was investigated. The inhibition of mesocotyl and cell elongation by BL was restored by application of exogenous GA(3) in har1. The endogenous GA(3) level correspondingly decreased in har1 mesocotyl especially from 1 to 4 h after BL irradiation. Putative genes of GA metabolism enzymes SbGA20ox, SbGA3ox and SbGA2ox were detected by Real-Time PCR and the results showed that one of the SbGA2ox homologs appeared significantly higher transcript level in har1 than in R111 at 2 h after BL irradiation. Putative homologous genes of DELLAs increased after BL irradiation and were higher in har1 among the three homologs. Remarkable increase of the DELLA expression was observed responding to exogenous paclobutrazol (PAC). Our research provided evidence in monocot sorghum, that the changes of a set of the GA metabolism and signaling genes might be involved in BL-induced inhibition of cell elongation.