Abstract: Accurately predicting pressure loss of spacer grids and sub-channels is critical for fuel assembly spacer grids and holding system design and for developing critical heat flux (CHF) correlation. The present study distinguished different types of sub-channels in a typical mixing spacer grid, and focused on the investigation of the relationship between spacer grids pressure loss and the sub-channels pressure loss. Different design features were considered, including springs, dimples, mixing vanes and guide tabs etc. Based on classic hydraulic resistance results, all design features were hydraulically equivalent to flat plate with an angle in a circular tube. Models of predicting pressure loss of spacer grids and sub-channels were established. It is found that the prediction results agree with the experimental results very well.