To improve antibody production in Chinese hamster ovary (CHO) cells, the humanized antibody-producing CHO DP-12-SF cell line was transfected with the gene encoding activating transcription factor 4 (ATF4), a central factor in the unfolded protein response. Overexpression of ATF4 significantly enhanced the production of antibody in the CHO DP-12-SF cell line. The specific IgG production rate of in the ATF4-overexpressing CHO-ATF4-16 cells was approximately 2.4 times that of the parental host cell line. Clone CHO-ATF4-16 did not show any change in growth rate compared with the parental cells or mock-transfected CHO-DP12-SF cells. The expression levels of mRNAs encoding both the antibody heavy and light chains in the CHO-ATF4-16 clone were analyzed. This analysis showed that ATF4 overexpression improved the total production and specific production rate of antibody without affecting the mRNA transcription level. These results indicate that ATF4 overexpression is a promising method for improving recombinant IgG production in CHO cells.