Rapid thermal annealing (RTA) has been performed on InGaAsP solar cells with the bandgap energy of 1 eV grown by molecular beam epitaxy. With the employment of RTA under an optimized condition, the open voltage was increased from 0.45 to 0.5 V and the photoelectric conversion efficiency was increased from 11.87-13.2%, respectively, which was attributed to the crystal quality improvement of p-type InGaAsP and therefore a reduced recombination current inside depletion region. The integral photoluminescence (PL) intensity of p-type InGaAsP increased to 166 times after annealing at 800 degrees C and its PL decay time increased by one order of magnitude. While the changes of nominally undoped and n-doped InGaAsP were negligible. The different behaviors of the effect of RTA on InGaAsP of different doping types were attributed to the highly mobile "activator" - beryllium (Be) atom in p-type InGaAsP.