The air-dried density, bending strength (MOR) and compressive strength parallel to grain of different grades of rotten wood were measured with test specimens of 100-130 year-old larch (Larix sp.) collected in Wu Ying Dian, the Forbidden City. In addition, the MOR of specimens was measured by Resistograph and the correlations were analyzed between resistance value and mechanical properties of wood. The correlations are significant for quantitative evaluation of wood structural condition by Resistograph. The results showed that wood air-dried density, strength properties and resistance values decreased significantly with the increase of decay degree, and there were remarkable differences among various grades of wood. Based on the rotten degree, the attenuation rate is in the following order: MOR> compressive strength parallel to grain > air-dried wood density > resistance value, and the above 4 indexes were 32%, 53%, 73% and 74% of those of intact wood, respectively, when the degree of rotten wood reached grade 3. Significant positive correlation at 0.01 level was showed between the wave trough values and wood air-dried density, MOR, compressive strength parallel to grain. The correlations between the wave peak values and the average value and air-dried wood density, MOR reached 0.01 level, and the correlations between the wave peak values and the average value and compressive strength parallel to grain reached 0.05 level. Based on the correlation between resistance value and physico-mechanical properties of wood, the values could be analyzed quantitatively. The air-dried density, bending strength (MOR) and compressive strength parallel to grain of different grades of rotten wood were measured with test specimens of 100-130 year-old larch (Larix sp.) Collected in Wu Ying Dian, the Forbidden City . In addition, the MOR of specimens was measured by Resistograph and the correlations were analyzed between resistance value and mechanical properties of wood. The correlations are significant for quantitative evaluation of wood structural conditions by Resistograph. The results showed that wood air-dried density, strength properties and resistance values ​​decreased significantly with the increase of decay degree, and there were remarkable differences among among various grades of wood. Based on the rotten degree, the attenuation rate is in the following order: MOR> compressive strength parallel to grain> air- dried wood density> resistance value, and the above 4 indexes were 32%, 53%, 73% and 74% of those of intact wood, respectively, when the degree of r otten wood reached grade 3. Significant positive correlation at 0.01 level was showed between the wave trough values ​​and wood air-dried density, MOR, compressive strength parallel to grain. The correlations between the wave peak values ​​and the average value and air-dried wood density, MOR reached 0.01 level, and the correlations between the wave peak values ​​and the average value and compressive strength parallel to grain reached 0.05 level. Based on the correlation between resistance value and physico-mechanical properties of wood, the values ​​could be analyzed quantitatively .