A new equation of particle diffusion coefficient in turbulence was presented. It was compared with numerical simulation of particle diffusion in turbulence and the results calculated with Hinze- Tchen equation and Govan equation The equation agreed with the numerical simulation very well on the conditions that the relaxation time of particle is greater then the turbulent characteristic time scale and the settling velocity of particle is less than the fluctuating velocity of turbulence. The Hinze- Tchen equation was applicable only when the relaxation time of particle was less than the turbulent characteristic time scale

and the Govan equation was useful only when the settling velocity of particle was much greater than the fluctuating velocity of turbulence.