In the field of mathematical optimization, stochastic dynamic programming (SDP) is a good framework to model uncertain optimization problems. In power system, demand dispatch (DD) is a new emerging operation concept compatible with uncertain renewable generations. In the conventional operation concept called load following, generation follows the load variations through economic dispatch program which is actually supply dispatch. But, to operate a system with a high penetration of stochastic generation units, it would be better to apply DD with the paradigm of generation following. In this paper for the first time we seek to present DD problem through clear formulations based on SDP of dispatchable loads. If the operation horizon, here a day, is divided by N time periods, the operator tries to find the chain of optimal control law so as to minimize the expected total cost. The operator selects the optimal control strategy for every time period.