The thermal stability of B-doped SiGe layers formed on Si substrates is studied, using samples prepared by Si-GeH4-B2H6 molecular beam epitaxy. Strain relaxation and the diffusion of Ge and B in the samples are measured after heat treatment corresponding to heterostructure bipolar transistor (HBT) fabrication processes. The strain in the SiGe layer remains almost constant during annealing at up to 950°C for 30 min, while some misfit dislocations are formed above 800°C in samples with relatively high Ge content (>10 at.%). Ge diffusion is negligible up to 950°C, but B atoms diffuse considerably even if the temperature for 30 min annealing is as low as 850°C. Accordingly, the formation of misfit dislocations and B diffusion are the dominant factors limiting the Ge content and thermal treatments in HBT processes.