In order to culculate the critical speed of propeller shaft, the accurate value of virtual moment of inertia of marine propeller should be estimated. Virtual moment of inertia consists of the mass moment of inertia of propeller I and the additional moment of inertia by entrained water Δ I. It is usual to estimate Δ I by making a percentage additon to I, varying from some 20 percent to 30 percent, but this estimate is sometimes accompanies by risk, The auther introduced the theoreitcal formula by application of two dimensional, and three dimensional potential flow theory, and by the test results for series model propeller the coefficient of the theoretical formula were added some correction the simple practical formula for Δ I were obtained. The availability of this practical formula was also prove by the test results of torsional vibration of propulsion at sea trial,