This paper proposes a rate equation of contact nucleation caused by crystal-impeller collision J(c-i) = k' . p . n(s) . d(p)(N) . u(N+1) . D-b(-4), where k' is the coefficient, p the impeller blade number, n(s) the crystal number, d(p) the crystal size, u the tip speed of an impeller, and Db the impeller blade size. Contact nucleation rates of potassium alum measured in three stirred tanks of different size (1.57 x 10(-3), 6.28 x 10(-3), 5.02 x 10(-2) m(3) of working volume, P = 4, 6, n(s) = 10-2000, d(p) = 3.585 x 10(-4)-1.545 x 10(-3) m, u = 0.817-2.136 m/s, D-b = 9.45 x 10(-3), 1.5 x 10(-2), 3.0 x 10(-2) m) were successfully explained by this equation when the value of an unknown parameter (N) was equal to 3. The meaning of N = 3 is also discussed using the measured frequencies of particle-impeller contact (non-nucleation experiments).