This paper develops theoretical delay models for protected left-turn operations at a pretimed signalized intersection during heavy traffic. When the through traffic demand is heavy, residual queues from the previous cycle may occur and elevate the probability of blockage to the left-turn bay, leading to increased delay for left turns. A probabilistic left-turn delay model based on the queuing diagram is proposed for a leading left-turn operation; the influence of residual queues and blockage by the through traffic are taken into account. When the left-turn demand becomes heavy, the left turns may spill back to block the through traffic, resulting in through traffic delays. Through traffic delay is modeled probabilistically on the basis of the analysis of left-turn bay spillback for a lagging protected left-turn operation. The left-turn delay models are validated through carefully designed simulation studies using VISSIM and the results are compared with those from the Highway Capacity Manual (HCM) delay model, which does not consider blockage or spillback situations. The proposed delay models can be used to replace the uniform delay term in the HCM model for high-demand situations when the left-turn operation is affected by spillback and blockage.