The carbon-incorporation behavior in phosphorous-doped Si1−yCy/Si (y1∼0.018, y2∼0.024) epilayers grown by reduced pressure chemical vapor deposition (RPCVD) has been investigated as a function of annealing temperatures. An abnormal interstitial carbon (Ci) re-incorporation was observed in the initial stage of thermal annealing, introducing an additional tensile strain into the Si1−yCy epilayers. At higher temperature but below β-SiC precipitation threshold, almost complete strain relaxation was found. These strain transitions can be attributed to the competitive behavior between Ci re-incorporation and phosphorus deactivation to kick out the substitutional carbon (Csub) atoms during the post-annealing process. This work demonstrated that Si1−yCy epilayers grown by RPCVD could keep both enhanced carbon incorporation and nonequilibrium phosphorus activation if the thermal budget is well controlled.