We have achieved a significant improvement in the crystal quality of ZnSe films grown by pulsed molecular beam epitaxy (MBE) on Si(lll) by irradiating the substrates with a plasma of nitrogen (N-plasma) prior to the deposition. Reflection high-energy electron diffraction (RHEED) patterns during the pulsed MBE growth on the N-plasma-treated Si surface showed very well defined streaks with a twofold reconstruction indicating an atomically flat surface. In sharp contrast spotty RHEED patterns with a diffuse background were observed during the initial stages of the conventional MBE growth of ZnSe on untreated substrates, indicating an initial three-dimensional growth mode. Atomic force microscopy confirmed a smoother surface for the samples grown on the N-plasma-treated Si substrates. Moreover, transmission electron microscopy revealed a decrease in the density of crystal defects in the ZnSe epilayers by the use of the N-plasma treatment.