Chlamydia trachomatis is a kind of obligate intracellular bacterial pathogen that causes ocular and sexually transmitted diseases. In this study, we analyzed the codon usage patterns of the C. trachomatis mouse pneumonitis biovar (MoPn) and Homo sapiens. We found large differences between MoPn and human codon usages. To enhance the expression of Chlamydia protein in mammalian cells, the DNA sequence encoding the major outer-membrane protein (MOMP) of MoPn was modified to substitute the human-preferred codons for rarely used codons. The huma-optimized MOMP gene was synthesized and cloned into the pcDNA3 vector, as was the wild-type MOMP gene. The protein expression levels of the human-optimized MOMP and wild-type MOMP genes were compared. The experiments showed that the human-optimized MOMP gene produced significantly higher levels of MOMP protein than the wild-type MOMP, both in vitro and in vivo, but no obvious difference was observed in the levels of modified and native MOMP mRNA expression. The immunogenicity of the 2 constructs was examined using BALB/c mice following intramuscular immunization. The results showed that the mice immunized with the human-optimized MOMP produced higher levels of antigen-specific IgG antibody and showed stronger delayed-type hypersensitivity reactions and proliferative T cell responses than those immunized with the wild-type MOMP. Antigen-specific stimulation of spleen cells obtained from human MOMP DNA immunized mice produced higher levels of interferon-gamma than those obtained from wild-type MOMP DNA immunized mice. Taken together, the data show that human-optimized codon optimization can significantly enhance the gene expression and immunogenicity of the C. trachomatis MOMP DNA vaccine.