In order to rapidly predict the frequency response function (FRF), endmills are divided into two parts, tool shank and fluted portion. Three equivalent models based on the principle of equal mass, equal area of cross-section and equal stiffness are proposed for the fluted portion, and the receptances can be easily obtained by using Timoshenko beam for both tool shank and fluted portion. Endmill’s FRF is shaped by coupling the FRF of individual part. The variation of relative errors from the three equivalent models is analyzed on these four factors, area moment of inertia of the cross-section, helix angle, percent of fluted portion lengh to tool length and ratio of tool length to diameter. The results also explain the fact that the equal stiffness method can achieve the best prediction accuracy for endmill A, B,C while the equal mass method is best for endmill D. Compared with harmonic analysis, the equivalent model method of endmills is computationally cheap while has an acceptable accuracy.