Abstract: A new type of gravity model of cities, F(r,τ)=GR ij (τ)r -D ,was deduced from Reilly-Converse's model, I ij =GQ iQ jr -b ,through introducing time function,Q=f(t),and time-delay parameter,τ,to the latter,and in the former, R ij (τ)=∫f i(t)f j(t+τ) dt is a correlation funtion.The gravity coefficient,G,was defined as the maximum correlation coefficient of the variables Q i and Q j ,and the exponent,b,proved to be a generallized fractal dimension.