The present study was aimed to investigate the molecular mechanisms responsible for the pathogenesis of severe factor XIII (FXIII) deficiency. Site-directed mutagenesis was conducted to obtain human FXIIIA expression plasmids bearing the mutations. Wild type FXIIIA recombinant plasmid (pcDNA3.1-FXIIIA-wt) and 2 mutant FXIIIA recombinant plasmids (pcDNA3.1/FXIIIA/77mut, pcDNA3.1/FXIIIA/174mut) were transfected into the cultured COS-7 cells using lipofectamine 2000 transfection reagent, respectively. FXIII activities were measured by the Berichrom(®) FXIII chromogenic assay. The expression levels of FXIIIA mRNA were detected by real-time RT-PCR. The recombinant FXIIIA mutants were determined by using Western blot and ELISA. The results showed that the normalized mRNA levels of 2 mutants in transfected COS-7 cells were 0.82 ± 0.21 and 0.76 ± 0.17, respectively. The relative levels of both mRNA transcripts were not significantly decreased as compared with the wild type (1.06 ± 0.51). FXIII activity and FXIIIA antigen levels in concentrated media of cell expressing the wild type protein were (24.0 ± 2.9)% and (13.2 ± 2.3)%, respectively. FXIII activity and FXIIIA antigen levels in cell lysates containing the wild type recombinant protein were (61.6 ± 30.4)% and (32.8 ± 14.5)%, respectively. However, the antigen levels and activity of 2 mutants were severely decreased as compared to the wild type. It is concluded that both mutations severely disturb the normal expression of FXIIIA protein. The reduction of expression levels and decreased activities of the 2 mutants provides a convincible explanation for the deficiency phenotype in the index case.