The modern distribution system with proliferated distributed energy resources (DERs) integration and increased demand response capacities is expecting to experience a deregulation process just as what is happening in the transmission system. Ex-post real-time distribution system locational marginal pricing (DLMP) is proposed in this paper as an effective economic signal to reflect the marginal cost of supplying incremental loads in different connecting points of the distribution system while considering actual real-time behaviors of DERs and residential consumers. DLMP is defined for each phase at each bus. An incremental linear optimization problem is built by linearizing nodal current injection-based system state equations at the current system operation point, which is provided by the distribution system state estimator. The objective of the incremental linear optimization problem is to maximize the incremental distribution system social welfare, while constrained by a set of incremental system state equations, power balance equations, and adjustment limits of flexible generations/loads. Ex-post real-time DLMPs are calculated via the optimal dual solutions of the incremental linear optimization problem. A modified IEEE 13-bus distribution system is adopted to evaluate the performance of the proposed model.