By using all available times of light minimum, the orbital period changes of a W UMa eclipsing binary YY CrB have been investigated. Two possible orbital period variations are found: (1) a secular period decrease with a rate of dP/dt = -6.727 × 10-7 d yr-1, on which a sinusoidal oscillation with a period of P = 12.68 yr is superposed, or (2) an abrupt period change with a sudden period decrease of -0.7862 s around the year 2005. The secular period decrease is discussed in terms of (1) the mass transfer from the more massive companion to the less massive one with a mass transfer rate of dM2/dt = 2.8096 × 10-7 M⊙ yr-1, and (2) the mass loss via stellar wind from the system. The sinusoidal oscillatory can be interpreted as a result of the magnetic activity cycles of the system. In addition, the abrupt period decrease of the system might be due to the sudden mass transfer between the components.