Reliability-based design optimization(RBDO) is intrinsically a double-loop procedure since it involves an overall optimization and an iterative reliability assessment at each search point.Due to the double-loop procedure, the computational expense of RBDO is normally very high.Current RBDO research focuses on problems with explicitly expressed performance functions and readily available gradients.This paper addresses a more challenging type of RBDO problem in which the performance functions are computation intensive.These computation intensive functions are often considered as a“black-box”and their gradients are not available or not reliable.On the basis of the reliable design space(RDS) concept proposed earlier by the authors, this paper proposes a Reliable Space Pursuing(RSP) approach, in which RDS is first identified and then gradually refined while optimization is performed.It fundamentally avoids the nested optimization and probabilistic assessment loop.Three well known RBDO problems from the literature are used for testing and demonstrating the effectiveness of the proposed RSP method. Reliability-based design optimization (RBDO) is intrinsically a double-loop procedure since it involves an overall optimization and an iterative reliability assessment at each search point. Due to the double-loop procedure, the computational expense of RBDO is normally very high. Current RBDO research focuses on problems with explicitly expressed performance functions and readily available gradients. This paper addresses a more challenging type of RBDO problem in which the performance functions are computation intensive. The computation intensive functions are often considered as a “black-box” and the gradients are not available or not reliable. On the basis of the reliable design space (RDS) concept proposed earlier by the authors, this paper proposes a Reliable Space Pursuing (RSP) approach, in which RDS is first identified and then gradually refined while optimization is performed. It fundamentally avoids the nested optimization and probabilistic assessment loop. Well known known as RBDO pro blems from the literature are used for testing and demonstrating the effectiveness of the proposed RSP method.