The deformation behavior of Si–B–C–N ceramics derived from a polymer precursor has been investigated by compression tests at high temperatures. The hot isostatic pressing of pyrolyzed powder compact was conducted at 1450^°C and at 900 MPa in order to obtain a dense ceramic monolith. The material consisted of the amorphous matrix and the dispersed Si₃N₄ particles with the diameter of about 20 nm. While only apparent elastic deformation was observed at the testing temperatures up to 1500^°C, significant plastic deformation occurred at temperatures higher than 1650^°C. The plastic deformation is considered to be caused by the viscous flow of the amorphous matrix. The flow stress at the testing temperatures in the range of 1650 to 1750^°C increased with initial strain rate. The flow stress was not proportional to the strain rate. The shear viscosity at 0.02 strain was about 10¹¹–10¹³ Pa s and decreased proportionally to about −0.75 power of the strain rate. The decrease in shear viscosity is considered to show a strain localization behavior. Strain hardening was observed during the plastic deformation. As the reason of the strain hardening, the crystallization can be considered, in addition to the decrease in atomic site defects during the deformation.