The purpose of this study is to analyze the temperature effect on the infinite multiplication factor of light water moderated UO2 lattices KRITZ: 2-1 and KRITZ: 2-13, and UO2-PuO2 lattice KRITZ: 2-19 of the Sweden KRITZ reactor. To quantify the contribution of each component of the infinite multiplication factor to the temperature coefficient, calculations were performed for the three configurations of KRITZ benchmark at two temperatures: room temperature 20°C and an elevated temperature 245°C, using MCNPX which is a continuous energy Monte Carlo reference code for the calculations of neutron transport. Nuclear data needed for this work were processed by means of the NJOY99 system and the most recent libraries JENDL-4, JENDL-3.3, JEFF-3.1, ENDF/B-VII, and ENDF/B-VII.1. The performed studies on the temperature coefficient have shown an agreement between our calculations and those obtained from the final report published by the OECD in 2005 (NEA, 2005).