A respirator protects its user by sealing the user's face and filtering hazardous particles from environment. However, faceseal leakages of users-respirators always happen. First, this study investigated a computer-aided technique for designing a well fitted N95 filtering facepiece respirator (FFR) for a subject. The customized N95 FFR includes a customized contact area, a center filtering area, two straps and a nasal clip. Five base contact areas of National Institution for Occupational Safety and Health (NIOSH) headforms were created and the customized contact area was modeled using the mapping relationship between the subject and a NIOSH headform. The center filtering area was designed by considering constraints of N95 FFR shape. Second, this study used simulation-based approaches, including the FE method and computational fluid dynamics (CFD) method, to assess the performance of the customized N95 FFR on the subject. The contact pressure and the faceseal leakage from the subject-customized N95 FFR combination were compared with the results from the subjectexisting N95 FFRs combinations. The comparison showed that the customized N95 FFR provided the subject an optimized contact pressure distribution and no faceseal leakage.