Background: 3D spheroid model were known to be a good model to study the multicellular apoptotic resistance in most cancer cell lines. We found that 3D spheroids of EGFR-mutant lung cancer cell showed more prominent apoptosis to tyrosine-kinaseinhibitor, erlotinib than 2D. BIM expressions before and/or after treatment of TKI were more prominent in 3D than 2D in several EGFR-mutant lung cancer cells. So we concluded BIM can explain the more apoptosis in 3D. But the other mechanisms of 3D sensitivity to TKI treatment are not studied yet. Methods: We used EGFR-mutant cell line, HCC4006 and A549 without EGFR mutation and generated 3D spheroids. 2D and 3D spheroids were treatment with erlotinib. The degree of apoptosis were compared between 2D and 3D. Also the BIM and TNF-related apoptosis-inducing ligand (TraiL) expression were compared. After finding of relatively elevated TraiL expression in 3D, we silenced TraiL by siRNA and compared the degree of apoptosis between 2D and 3D.Results: We found that only HCC4006 not A549 showed apoptosis and elevated BIM expression after Erlotinib treatment. We found more elevated TraiL expression in 3D. So we assumed that TraiL is one of the possible mechanisms of more apoptosis to Erlotinib in EGFR-mutant lung cancer 3D spheroids. After silencing the TraiL by siRNA, difference in the degree of apoptosis between 3D spheroids and 2D to TKI disappeared. Conclusion: Adding to increased BIM, we can suggest that elevated TraiL expression can be one of the possible mechanisms of the more prominent apoptosis in 3D spheroids of EGFR-mutant lung cancer