High temperature deformation behavior of polymer derived Si-B-CN ceramics both in amorphous and crystalline state has exhibited interesting potential. It could be seen that the creep rate decreases continuously with time and the dependence of the strain rate,έ for the amorphous and nano-crystalline ceramics is rather similar, including the absence of any asymptotic behavior till 300 h of testing time. For the crystalline material the deformation rates are about one order of magnitude smaller than for the amorphous counterpart, which holds for the whole testing time. The deformation rates decreases by about 2 1/2 orders of magnitude reaching a value of 10-9 s-1 after 300 h at the end of the experiment. Newtonian viscosities of amorphous and nano-crystalline Si-B-C-N ceramics derived from load change experiments were found to be very similar with values of 1015 to 1016 Pa·s at 1400 oC. It was observed that the decreased strain rates in the crystallized materials as compared to the as-thermolyzed materials was mainly due to the densification as a result of annealing and structural changes in the matrix phase rather than due to the dispersion of the crystallites. © 2012 Nova Science Publishers, Inc. All rights reserved.