Aim To investigate expression of ubiquitin conjugating enzyme (Ubc E₂) and p53 in the apoptosis of human monocyte cell line U937 induced by oxidized low density lipoprotein (ox LDL). Methods U937 cells were incubated with ox LDL. The apoptotic cells were determined by DNA fragment analysis and flow cytometric analysis. The level of p53 and ubiquitin conjugating enzyme mRNA were quantified by reverse transcription polymerase chain reaction (RT PCR). The protein contents of ubiquitin conjugating enzyme, P53 and apolipoprotein B (apo B) were analyzed by immunofluoresence staining and flow cytometric method. Results The results showed that the increase of the degree of U937 cell apoptosis was concentration dependent. Ox LDL could down regulate the gene/protein expression of ubiquitin conjugating enzyme and up regulate the gene/protein expression of P53, meanwhile, the apo B accumulated in U937 cells. Conclusion The results implicated that ox LDL induced U937 cell apoptosis by accumulation of P53 and apo B through down regulating ubiquitin conjugating enzyme. Cellular defense system, either dependent or independent on the ubiquitin system, was weakened by the ox LDL induced toxicity.