Objective：To develop a simple and efficient method for constructing a middle fragment-deleted mutant of MHC class Ⅱ molecule transactivator （C ⅡTA ）mutant with the 109^th to 226^th amino acid codons deleted. Methods： Two gene fragments at each end of the deleted C Ⅱ TA gene were obtained by OE-PCR method and were mixed together for 8 PCR cycles without primers to achieve effective overlapping, then 2 primers was added for amplification of the desired fragments. The amplification products were subsequently cloned into eukaryotic vector pIRES for identification. Results： A mutant of C Ⅱ TA with the 109^th to 226^th amino acids deleted was successfully constructed. Conclusion： This modified OE-PCR technique overcomes some shortcomings of traditional method and is very suitable for constructing mutants with middle fragment deletion, making it worth to be popularized