Densification behavior by hot isostatic pressing in precursor-derived Si-C-N system ceramics has been investigated in order to obtain dense ceramic bulk derived from polymer precursor. In advance of the experiments of densification, the deformation behavior of as-pyrolyzed monolith at high temperatures has been investigated. The as-pyrolyzed ceramic monolith which had many pores showed deformation of several % strain in uniaxial compression tests at the temperatures higher than 1400°C. Then, hot isostatic pressing of both as-pyrolyzed monolith and pyrolyzed powder compacts has been performed for the densification. The density of the monolith increased more than 20%, and many pores disappeared during the hot isostatic pressing at 1600°C and 980 MPa. However, the size of residual pores became larger than the initial pore size before the treatment. On the other hand, dense ceramics were obtained by hot isostatically pressing the pyrolyzed powder compaction. In the case of the treatment at 1400°C, the microstructure maintained the amorphous matrix. These dense materials were able to be more deformed at 1600°C than the as-pyrolyzed monolith. Also, in case of the Si-C-N-B ceramics, similar dense materials were obtained by the techniques of hot isostatic pressing of powder compaction.