The four-point bending creep behavior of a Sm-alpha-beta Sialon composite, in which Sm-melilite solid solution (denoted as M') was designed as intergranular phase, was investigated in the temperature range 1260-1350degreesC and stresses between 85 and 290 MPa. At temperatures less than 1300degreesC, the stress exponents were measured to be 1.2-1.5, and the creep activation energy was 708 kJ mol(-1), the dominant creep mechanism was identified as diffusion coupled with grain boundary sliding. At temperatures above 1300degreesC, the stress exponents were determined to be 2.3-2.4, and creep activation energy was 507 kJ mol(-1), the dominant creep mechanism was suggested to be diffusion cavity growth at sliding grain boundaries. Creep test at 1350degreesC for pre-oxidation sample showed a pure diffusion mechanism, because of a stress exponent of 1. N-3-diffusing along grain boundaries was believed to be the rate controlling mechanism for diffusion creep. The oxidation and alpha --> beta Sialon phase transformation were analyzed and their effect on creep was evaluated.