This work presents an iterative look-up table based thermal simulator, I-LUTSim, to efficiently estimate the temperature profile of three-dimensional integrated circuits. I-LUTSim includes two stages. First, the pre-process stage constructs thermal impulse response tables. Then, the simulation stage iteratively calculates the temperature profile via the table lookup. With this two-stage scheme, the maximum absolute error of I-LUTSim is less than 0.41% compared with that of a commercial tool ANSYS. Moreover, I-LUTSim is at least an order of magnitude faster than a fast matrix solver SuperLU  for the full-chip temperature simulation.