This paper presents a numerical modeling approach for investigating crescent‐type internal gear pumps (CIGPs) considering radial micromotions of both gears. The modeling approach couples a lumped parameter pressure solver on the bulk fluid domain, with a transient computational fluid dynamics film simulator based on transient gap geometries solved from the gear loading forces. Simulation results on a commercial unit shows that for a wide range of operating conditions, the model gives volumetric efficiency predictions with errors less than 2 % comparing to measurements. To highlight the importance of considering radial micromotions, the paper also provides the results achieved assuming nominal position for both gears. A numerical approach for investigating crescent‐type internal gear pumps, coupling a lumped parameter pressure solver with a CFD film simulator based on transient gap geometries solved from gear loading forces was proposed. Volumetric efficiency prediction was validated by measurements. The ring gear film leakage was identified as the major volumetric loss source in the unit by the model.