The effect of osmotic condition on β-glucuronidase (GUS) transient expression was evaluated in microspore-derived embryos of wheat. Microspore explants were treated on medium containing various mannitol concentrations prior to and post bombardment with plasmid DNA pAHC25 containing uidA gene controlled by maize ubiquitin 1 (UBI1) promoter. GUS expression in the bombarded explants was examined by histochemical and fluorometric assays. Increased GUS expression was observed with mannitol treatment when compared to untreated explants. The histochemical study showed that the number of blue (GUS) foci were the highest in the bombarded explants treated with 0.6 M mannitol medium. The fluorometric assay of bombarded explants also proved 3.5-fold increase in GUS activity with 0.6 M mannitol treatment when compared to without mannitol treatment. These results indicate that 0.6 M mannitol is beneficial for improving transformation efficiency of wheat microspore-derived embryos or embryogenic calli through biolistic transformation.