Background: Use of contrast agents (CAs) during CT simulation for treatment planing system leads to changes in electron density. In this study we aimed to investigate the effect on calculated dose of various concentrations of CAs on treatment planing systems in different dose calculation algorithms. Materials and Methods: Contrast agent (0.769 mg/ml Iopromid) - water mixtures at concentrations of 0%, 1%, 2%, 5% and 10% in total volume of 500 ml for each were made by using five identical balloons. Calculations were performed by Cobalt-60 and 10 MV linear accelerator devices in CMS XIO treatment planning system. The prescribed dose of 100cGy was given to the center of balloon that is isocenter of SAD technique at 10cm from the surface. The doses at maximum dose depth (dmax) and at 5 cm were calculated according to the separate algorithms by either making or not making a correction for CA, and the results were recorded. Results: In all algorithms, as contrast ratio increases, the dose values at dmax and 5 cm-depth increase accordingly. When the doses at dmax and 5 cm-depth were compared for Linac and Co-60 in all algorithms, it has been shown that the dmax value of Co-60 was higher and the difference was greater in parallel with increasing contrast ratio in comparing with Linac. Conclusion: When required during the planning, the treatment plan should be calculated via providing an electron density correction by contouring the volume retaining CAs along the beam line.