Thermally modified wood has in recent times become more and more starting to receive into the awareness and more frequently used in our country, and therefore, it is important to know its characteristics in terms of flammability and fire protection and safety. Diploma thesis deals with the assessment of the impact of thermal modifications of wood forest pine (Pinus syslvestris L.). The samples were thermally treated at temperatures of 160 °C and 200 °C and exposure time of 3 and 9 o'clock. The samples were tested by the method of cone-calorimeter in the meaning of standards ISO 5660-1 (1997) and their relative flammability compared on the basis of the evaluation criteria, which are: heat release rate HRR, maximum average rate of heat emission MARHE, the total amount of heat released during the test, the average rate of loss of weight during the test and efficient heating value. For best results when all evaluation criteria amounted to thermally treated samples at a temperature of 200 °C and exposure time of 9 hours compared to the samples without thermal modifications. In this way the modified samples were measured by the value of the maximum rate of heat release 290,75 kW/m2, the maximum average rate of heat release 210.06 kW/m2, the total amount of heat released 161.07 MJ/m2, the average rate of loss of weight during the test 6,34 g/(m2.s-1) and efficient heating value 17,33 MJ/kg.