The variations of the orbital period of the near-contact binary system DD Monocerotis were analyzed based on three new CCD times of light minimum together with others compiled from the literature. The general trend of the ($O-C$)$_1$ curve indicates a continuous period increase at a rate of $dP/dt$$=$$+$1.40 $\times$ 10$^{-7}$d yr$^{-1}$. Meanwhile, by using all available photoelectric and CCD data, a small-amplitude period oscillation ($A$$=$ 0.0039 d; $T=$ 18.8 yr) was found to be superimposed on the period increase. The continuous period increase is explained by mass transfer from the less-massive component to the more-massive one ($dM_{2}/dt$$=$$+$6.99 $\times$ 10$^{-8}M_{\odot}$yr$^{-1}$), which is consistent with the semi-detached configuration of the binary with a lobe-filling secondary discovered by Qian et al. (1997, A&AS, 125, 475). We showed that the timescale of the period change is close to the thermal time scale of the secondary, suggesting that DD Monocerotis is undergoing a case A mass transfer. The small-amplitude period oscillation reveals the presence of an extremely cool stellar companion ($M_3$sin${i^{\prime}}$$=$ 0.13$M_{\odot}$) in the system. Accumulation of photoelectric and CCD data will discover small-amplitude cyclic changes in many close binary stars in the future.