In this research, thermal properties of I x I rib fabrics knitted by using various yams of different properties were investigated with all details. The mentioned yarn properties were yam count, yam twist and combing process. The thermal resistance, thermal absorptivity, thermal conductivity, water vapour permeability of samples were measured with the aid of Alambeta and Permetest devices respectively. The results of the tests were evaluated statistically and the importance levels of the relationship between the measured parameters were determined. It is observed that yam properties like yam count, yam twist and combing process of cotton have affected different thermal comfort properties of I x I rib knitted fabrics. While the yam twist and yam count increase thermal resistance values decrease and water vapour permeability values increase. The combing process has the same effect on the thermal properties. (C) 2007 Elsevier Masson SAS. All rights reserved.