Abstract: The 3-D flow field of rod bundles with spacer grids and the effects of grid structure on the flow are studied by numerical solution with computational fluid dy namics(CFD) method and laser Doppler velocimeter test. Velocity profiles from the CFX program were compared with laser Doppler velocimeter measurements taken from the flow field upstream and downstream of spacers in a full scale, cold water test loop. The results of the comparison show good agreement.