The article presents the main results of experimental studies on the identification of the main grade defects of oak lumber by the thermal non-destructive testing method. Regressional dependences of wood defects temperature display from the main factors for the studied grade defects are proposed. Indicators of infrared radiation (temperature range) of the main visible oak grade defects obtained as a result of experimental studies are presented. A conceptual scheme for the line control methods of identification of the main grade defects in lumber are proposed.