The paper deals with sap blue stains of pine woods. The result shows that the colour change of latewood caused by both Trichoderma koningii and Ceratocystis sp. are earlier and greater than earlywood. We concluded that the corelationship between free radical contents and stain degrees is evident by the study of ESR. The sap stain of pine woods are closely related to such elements as resin canal, wood ray, extractives, water content etc.