We study D → ππ decays with final state interactions considered in one-particleexchange method. A clear physical picture for final state interactions based on quark and hadronic level diagrams is presented. A strong phase is introduced for hadronic effective couplings, which is crucial for explaining the experimental data of D+ → π+π0, D0 → π+π−, and D0 → π0π0. Rescattering effects between different D decay channels are usually large. They are important for obtaining correct branching ratios for D → ππ decays in theoretical calculation.