The aim of this study is to investigate the effect of sepiolite mineral used in the production of polypropylene based wood plastic composite (OPK) materials on thermal properties and thermal conductivity. The pellets obtained from the mixture of wood flour, polypropylene and sepiolite minerals with double-screw extruder were milled and pressed to produce OPK boards. Thermal properties such as Thermogravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC) as well as the thermal conductivity properties of the produced boards (ASTM C 1113-09) have been determined. It has been determined that the addition of the sepiolite mineral used as fire retardant in OPK materials increases the thermal and thermal conductivity properties, while the crystallization degree decreases.