In order to understand the mechanism of temperature dependence of flow stress in high-temperature deformation of pure copper, dislocation behaviors during deformation were investigated by using in-situ time-of-flight neutron diffraction. The dislocation density was determined by line-profile analysis of the neutron diffraction data. Based on the dislocation density, the flow stress was estimated using the Bailey-Hirsch equation, and the relationship between the mechanical properties and the dislocation density was discussed. The increase in the dislocation density during the deformation was suppressed with increasing temperature. It can be deduced that dislocation recovery with annihilation was more likely to occur at higher temperatures. Estimation of the flow stress using the measured dislocation density reproduced well the temperature dependence of the flow stress. It was also confirmed that the oscillation of flow stress was caused above a certain temperature by the alternating increase and decrease of dislocations in the range of 10¹²-10¹³ m^-2. The decrease in ductility as the temperature increases to about 400°C corresponds to the decrease in dislocation strengthening per unit length along with the decrease in dislocation density.