We have collected optical observational data of the blazar PKS 2251+158 for nearly 100 years and analyzed its long-tem periodicity in the light variation. The maximum amplitude of light variation in B was found to be 2.86 mag. Using the Jurkevich method, a period of 12.39 yr was found. It is probable that at the center of this object there is a black hole of mass 8.0x10 5 and that the γ amplification factor is not less than 7.2. The Jurkevich method was also discussed.