Abstact:Objective: Epstein-Barr virus (EBV) latent membrane protein 1 (LMP1) is known to plays important role in B cells growth and transformation. LMP1 is considered to be a functional homologue of the CD40 receptor and they can activate many overlapping signaling pathways. In this study, we compared the function of CD40 with that of LMP1 in B cells transformation. Materials and methods: Expression of CD40L was observed in infected B cells with LMP1 mutated EBV. To observe the expression reverse transcription-PCR were performed. Results: This expression of CD40L did not support proliferation and transformation of B cell. Even in vitro proliferation and transformation of B cell infected with LMP1 deleted EBV supplemented with CD40L were also not observed. Conclusion: Despite many similarities shared between CD40 and LMP1, CD40-CD40L interaction didn't complement on LMP1 mediated B cells transformation in vitro.