Background and objectives: Nowadays, landfilling is most common method in many countries owing to lower cost and adaptation to wide range of solid waste. Site selection of landfill requires evaluating several parameters such as municipal government requirements, environmental regulations and a large number of quantitative and qualitative criteria. The aim of current study was to compare AHP and TOPSIS in landfill site selection. For this purpose, two mentioned methods were applied to select suitable site in Karaj. Materials and methods: In present study, 4 candidate sites in south of Karaj were selected for landfill by overlaying data layers (digital maps) and query functions in Arc GIS 9.2. Prioritizations between alternatives were conducted by AHP and TOPSIS technique according to the criteria mentioned. Eventually, we compared and evaluated the AHP results and TOPSIS results with each other. Result: According to AHP, site prioritization was 3,2,4,1 respectively whereas, in the case of TOPSIS, it was ranked 4,3,2,1, respectively. These results showed that both methods are suitable to determine site priority. Conclusion: As in AHP, alternatives are compared with respect to goal and criteria, consequently it has better precision and higher accuracy and confidence compared with TOPSIS.