The flat-walled diffuser/nozzles are classified into two types, i.e. Tube I and Tube II, based on their different flow resistance characteristics. Tube I has less pressure loss coefficients in the diffuser direction than the nozzle direction, and Tube II has larger pressure loss coefficients in the diffuser direction than the nozzle direction. This work focuses on the characterization of the diffuser efficiency, and flow rectification of these two types of diffuser/nozzles. The characterization is performed with diverging angles in the range of 5°–60° and length–width ratios in the range of 1–20, and the pressure drop ranging from 1 to 10 kPa. The results show that with the increase in pressure drop and the decrease in length–width ratio, Tube I type of diffuser/nozzles can change to Tube II. For Tube I type of diffuser/nozzles, the smaller the diverging angle and the longer the length, the better the performance. For Tube II type of diffuser/nozzles, the larger the diverging angle and shorter the length, the better the flow rectification performance. Simulation results match well with the experiment data. Of particular interest, simulation of the diffuser flow fields suggests that the flow separation has a significant impact on pressure loss coefficients in the diffuser direction.