We aimed to examine the effect of pioglitazone on transdifferentiation of preosteoblasts from rat bone marrow mesenchymal stem cells (BMSCs) into adipocytes and investigate its effect on bone metabolism. BMSCs were harvested from the femurs and tibias of a rat, then separated, purified, proliferated for 3 generations and differentiated into preosteoblasts for 5 days and 14 days respectively in the presence of osteogenic medium. Thereafter, the preosteoblasts were cultured for 21 days in the presence of adipogenic medium with and without pioglitazone (1 μg/mL). Partially-differentiated osteoblasts were identified by mineralized nodules with Alizarin red S staining. Transdifferentiated adipocytes were identified by Oil Red O staining. Reverse transcription PCR (RT-PCR) was performed to assay the expression levels of osteogenic markers Runx2 and ALP, and an adipogenic marker PPARγ. Those cells cultured for 5 days did not show mineralized nodules as detected by staining of Alizarin red S, while those cultured for 14 days showed dispersed mineralized centers in the form of brown spots, although without obvious red mineralized nodules. After adipogenic transdifferentiation for 21 days, adipose-drops were found in cells of 5CG and 5EG earlier than those of 14CG and 14EG, and the former showed much more adipocytes separately as detected by Oil Red O staining. Whatever the time was 5 days or 14 days of BMSCs osteogenic differentiation, the cells cultured with pioglitazone showed much more adipocytes than those without pioglitazone. Our experiment showed that the less time it took for BMSCs osteogenic differentiation, a stronger ability remained for BMSCs to transdifferentiate into adipocytes. The mRNA expression levels of Runx2 and ALP were decreased by 1.79 and 1.90 times respectively in 5EG (P< 0.05) as compared with 5CG, and that of PPARγ was increased by 1.31 times in 5EG (P<0.05) as compared with 5CG. The mRNA expression levels of Runx2 and ALP were decreased by 1.45 and 1.54 times respectively in 14EG (P<0.05) as compared with 14CG, and that of PPARγ was increased by 1.39 times in 14EG (P<0.05) as compared with 14CG. It was concluded that pioglitazone stimulated the transdifferentiation of BMSCs into adipocytes. These observations provided a potential mechanism of imbalance in thiazolidinedione induced bone metabolism.