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The drill resistance of tree micro drill instrument can not be measured directly, but can be expressed indirectly by the current, voltage, power, speed and other parameters of DC motor. In order to improve the measurement accuracy of drill resistance, the expression method of the drill resistance was further studied. The principle of the micro drill instrument was described, and the relationship between the drill resistance and the current, voltage, power and speed of the DC motor controlling the drill rotation speed was analyzed, and six drill resistance expression methods were putted forward. The test wood samples included hardwood, softwood and conifer. The absolute dry wood density of each sample was measured. The drill resistance data was tested by self-developed micro drill instrument and Resistograph 650-S produced by German Rinntech Company. Average resistance value expressed by each resistance expression method of self-developed micro drill instrument and average resistance value measured by Resistograph 650-S were calculated to each test sample. The linear models between each average resistance value and the absolute dry wood density were established. The results showed that the linear correlation coefficient between DC motor current, voltage, power and absolute dry wood density were higher than 0.9, and the current, voltage and power of DC motor can be used to express the relative magnitude of drill resistance;the linear correlation coefficient between DC motor voltage adjusted by rotation speed of DC motor and absolute dry wood density was the highest, so when measuring absolute dry wood density, the drill resistance expressed by DC motor voltage adjusted by rotation speed of DC motor was more appropriate in the six drill resistance expression methods. The linear correlation coefficient of self-developed micro drill instrument between DC motor voltage adjusted by rotation speed of DC motor and absolute dry wood density was 0.0053 higher than that of the German Resistograph 650-S, which indicated that the design scheme and the drill resistance expression method were reasonable. The design scheme and the drill resistance expression method proposed had great reference value for the independent production of micro drill instrument in China.