Optimized antigen expression is critical to the immunogenicity of DNA vaccines. A number of approaches have been proposed to enhance the antigen expression and/or immunogenicity of DNA vaccines, but their relative contributions have not been compared in a same antigen system. In the current study, optimization of codon usage, enhancement of viral promoter function and selection of secretary leader sequences were evaluated for their roles in improving the immunogenicity of a same model antigen, the HIV-1 envelope glycoprotein. Our data demonstrated that all these factors can work synergistically to improve the final antigen expression and immunogenicity of HIV-1 Env DNA vaccines, indicating they work through different mechanisms. The best result came from the approach that optimized all three components in a DNA vaccine design. Our study further revealed that the levels of HIV-1 env-specific RNA transcripts in transiently transfected 293T cells were higher from the codon-optimized gene than the wild type counterpart. This finding suggested other mechanism may also contribute to the increased antigen expression and immunogenicity of codon-optimized DNA vaccines in addition to the improved tRNA usage in mammalian cells for codon-optimized viral genes as previously reported.