This work investigated the regulated emission characteristics of a biodiesel-fueled direct-injection (DI) engine with the addition of n-heptane, dimethoxymethane (DMM), and ethanol from the intake port on a single-cylinder engine. The effects of physical and chemical properties of the premixed fuel, partial equivalence ratio of biodiesel, and premixed ratio on emissions were evaluated. The experimental results revealed that the overall heat release curve exhibited a three-stage combustion with the addition of n-heptane but only one-stage with the addition of other two compounds. A simultaneous reduction of NOx and smoke opacity was obtained with the premixed fuels, and ethanol premixing showed the most significant effects on the reduction of NOx and smoke opacity. For a fixed partial equivalence ratio of each premixed fuel, premixing n-heptane showed a larger NOx increase slope and premixing DMM showed a larger smoke opacity increase slope with the increase of the partial equivalence ratio of biodiesel. HC emission was mainly influenced by the premixed fuel properties but not the equivalence ratio, and ethanol premixing had larger HC levels than n-heptane and DMM premixed fuels. When the overall equivalence ratio was kept constant, both HC and CO emissions increased but NOx emissions decreased at first up to a critical premixed ratio. As the premixed ratio exceeded the critical value, NOx emissions started to increase gradually, CO emissions began to decrease, but HC emissions almost remained constant. Under the above conditions, smoke opacity always improved with the increase of the premixed ratio.