A period study of the semi-detached eclipsing binary system W Delphini based on the extensive series of minimum timings covering more than a century(109 years) indicates a cyclic(O-C) variation of the system. This variation can be explained as due either to (1) stellar magnetic activity cycles of the cool subgiant G5 secondary component of the binary with a subsurface magnetic field equals to 3 kG, or (2) a long-term orbital period increases with a rate of 1.68 × 10-8 day/cycle caused by a mass transfer rate of 4.9 × 10-8M⊙yr-1 from the less to more massive component modulated by a light time effect due to a hypothetical third body with period of 53.4±1.06 years. The former explanation is more recommended than the later one since the obtained third body mass value(M3=1.58 M⊙) is quite large but it can not manifest itself observationally and also it cannot be a white dwarf. In the contrary, from the magnetic activity point of view, the obtained characteristics are in good consistent when applying Applegate(1992) mechanism. However, further precise photometric and CCD observations for minima timings with brightness determinations are needed to confirm the present solution.