||There are two similar yet conflicting urban gradient functions: the housing rent gradient function and the population density gradient function. According to the former, housing rent is a decreasing function of distance from central business district, CBD. The decrease is needed to compensate for the increase of travelling expense for commuting back to CBD. If commuting cost is the only concern, population should distribute evenly irrespective of distance from CBD. Hence, the decreasing population density gradient function is a contradictory phenomenon. This paper attempts to offer a supply answer to the question by developing the structural density gradient function.