The purpose of this study is to investigate the processibility of 6421 bismaleimide (BMI) resin for resin transfer molding (RTM) process and the mechanical performance of 6421 BMI composites reinforced by carbon fiber three-dimension (3D) and five-direction braided preforms. The thermal properties of 6421 BMI resin were investigated using differential scanning calorimeter (DSC) and thermogravimetric analysis (TGA). According to these data, the processing parameters and curing rule of BMI for RTM were derived. The flexural test results showed that 3D and five-direction braided composites had higher flexural modulus than that of laminated composites while the flexural strength was lower than that of the laminated composites. The impact strength of 3D and five-direction braided composites is 30.1% more than that of laminated composites. The results of mechanical properties tests indicate that carbon fiber 3D and five-direction braided/BMI composites are suitable for application.