© 2019 IOP Publishing Ltd.In the presented study, we proposed a design approach based on optimization algorithm to obtain mode order conversion in photonic crystal (PC) structures and experimentally verify the numerical result of the corresponding approach. The finite-difference time-domain method is integrated with an evolutionary algorithm to develop the introduced design method. Here, we designed two different mode order converter PCs with ultra-compact sizes that transform a fundamental mode to a higher order mode. The first designed structure realizes a mode order conversion in free-space host medium, and the second structure performs the same operation in a PC waveguide with optimized region. Also, asymmetric light transmission effect in PC waveguide structure is investigated. Additionally, we performed microwave experiments to confirm the numerical results of mode order conversion. In conclusion, we demonstrated a novel design approach to obtain mode order conversion, which can be feasible to design diverse photonic structures in the future applications of nanophotonics.