The heated fuel injectors are designed to bring up fuel temperature so as to reduce HC and CO emissions during cold start. The heated injectors are similar to regular injectors except heaters are placed near the injector inlet and outlet. The heaters, which has the ability to regulate temperature at 180 °C, transform the thermal energy to heat up the liquid fuel through the injector body. The heated injectors are required to heat up fuel to the operating temperature (e.g., 120 °F or 48.9 °C) as quickly as possible and to maintain that fuel temperature for about three minutes. However, test results indicate it takes more than two minutes for the fuel temperature to reach the desired operating temperature. Objective of this work is to find out the mechanisms controlling the slow heating process through CFD analysis.The computational domain covers the whole injector, from inlet to exit, since the heaters located near the top and bottom of the injector. The injector valve is held fully open to meter the largest amount of fuel to simulate the worst (slowest) heating conditions. Transient conjugate heat transfer between the solid phase (injector housing and the needle) and the liquid phase (fuel) was taken into account to predict the heating process. CFD results indicate the slow heating process is a result of fuel vaporization near the heaters. Fuel vaporization not only absorbs the thermal energy for latent heat, but it also increases the thermal resistance between the fuel and the injector body. Effect of the overall void fraction of fuel vapor on heat transfer process is also studied.