Strain stability of phosphorus doped pseudomorphically strained Si:C alloy is investigated via high-resolution X-ray diffractometry, Fourier transform infrared spectroscopy, and Hall measurement. Significant strain relaxations are found under post-annealing treatment far below β-SiC precipitation threshold temperature, especially for the highest phosphorus doped case. Most of the substitutional carbon is retained and no further β-SiC formation can be found for all samples investigated. Volume compensation through gettering of interstitial atoms around substitutional carbon is considered as a probable mechanism for the observed strain relaxation. The strain relaxation effect can be further reduced with HF treatment prior to post-annealing process. We found an upper limit for ion implant dose (<1 × 1014 atom/cm 2) for the retention of strain stability in phosphorus doped Si:C.