Although glucocorticoids (GCs) have been used to treat acute lymphoblast leukemia (ALL) for decades, the mechanisms of GC sensitivity and resistance in ALL cells are poorly understood. This study investigated the role and mechanisms of pro-apoptotic protein BIM in apoptosis of GC-sensitive and- resistant ALL cells. The dramatic apoptosis was observed in GC-sensitive CEM-C7 cells after incubated with DEX for 48 h, while not in GC-resistant CEM-C1 cells. The significant up-regulation of BIM in CEM-C7 cells induced by DEX was also observed, but no up-regulation of BIM was detected in DEX-induced CEM-C1 cells. When treated with DEX plus RU486, a glucocorticoid receptor blocker, the apoptosis and BIM expression of CEM-C7 cells were canceled. P38MAPK-blocking pharmacon SB203580 also significantly inhibited the up-regulation of BIM in CEM-C7 cells. These suggested that the absence of BIM up-regulation is one of the important mechanisms of GC resistance, GC-GR conjugation is indispensible in both GC-induced apoptosis and up-regulation of BIM, and p38 MAPK signal pathway is also involved in this process.