A model for predicting mechanical properties of ultra-high temperature ceramics (UHTC) and composites within a wide temperature range is presented. A model can be useful for predicting the mechanical properties of UHTC composites under dynamic loading and thermal shock. Results of calculations taking into account the dependences nonlinearity of the normalized elastic moduli on homologous temperature T/Tm in the range of 0.2 - 0.62. are presented.. Residual stresses in ZrB2 composites reinforced with particles of refractory borides, carbides and nitrides after selective laser sintering (SLS) or spark plasma sintering (SPS) were predicted. It is shown that the fracture toughness KIC of UHTC increases at the sintering temperature in the range 0.45 – 0.62 T/Tm. The residual stress in the matrix of ceramic composites can differ on a sign due to difference between the thermal expansion coefficients of the matrix and inclusion phases. It is shown that the fracture toughness and the flexural strength of ZrB2 matrix composites can be increased by 25% by the introduction of inclusions of specially selected refractory strengthening phases. Dependence of the normalized strength of composites ZrB2–B4C on the logarithm of normalized strain rate can be described by a power law in the range of strain rates from 10^-3 to 10^6 1/s and temperatures from 295 K to ~1673 K. Results of simulation confirm that the technologies of SLS and SPS can be used for the production of UHTC composites with high values of the specific strength and the fracture toughness.