In this paper, the theoretical flow ripple of an external gear pump is studied for pumps of similar size using different numbers of teeth on the driving and driven gears. External gear pumps with three different types of tooth profiles are studied. Nondimensional flowrates and fluctuation coefficients of gear pumps are discussed. By using the formula, flowrates can be calculated accurately and efficiently. Results indicate that: in the case of the same displacement (except teeth number and tooth width, other parameters of the gear pump are the same) the gear pump flow pulsation decreases with the increasing of the teeth number. We also concluded that changing tooth profiles on the driving and driven gear can get different pulse amplitude of the flow ripple.