In day-ahead power markets, the calculation of locational marginal price (LMP) relies on the load forecasting results. It is well known that short-term load forecasting results always contain certain degree of errors mainly due to the random nature of the load. At the same time, LMP step change exists at critical load level (CLL). Therefore, it is interesting to investigate the impact of load forecast uncertainty on LMP. With the assumption of distribution of actual load, this paper formulates the probability mass function of the random variable LMPt, LMP at time t, and then proposes the concept of probability-based expected LMP. Two useful curves, deterministic LMP versus forecasted load and expected LMP versus forecasted load, are presented. The first curve is designed to help identify the trustworthy regions of traditional LMP-Load curve. The second curve is demonstrated to be smooth and therefore eliminates the step changes in deterministic LMP simulation. The proposed concept and method are illustrated on a modified PJM 5-bus system.