Nitrogen (N2) fixation by heterotrophic bacteria associated with sinking marine particles is expected to have an important contribution to marine N cycle, but a mechanistic understanding of its regulation and significance is still unknown. We develop a mathematical model for unicellular heterotrophic bacteria growing on sinking marine particles and can fix N2 under suitable environmental conditions. We find that the interactive effects of polysaccharide and polypeptide concentrations in particles, sinking speed of particles, water temperature, and surrounding O2 and concentrations determine the N2 fixation rate inside particles. Our model suggests that anaerobic processes, including heterotrophic N2 fixation, can take place in anoxic microenvironments inside sinking particles even in fully oxygenated marine waters. In contrary to photosynthetic N2 fixation that is more prevalent in low latitude waters, the spatial range of heterotrophic N2 fixation associated with sinking particles is more widespread. The modelled rates are similar to bulk rates measured in the aphotic ocean, and our study consequently suggests that particle-associated heterotrophic N2 fixation contributes significantly to oceanic N2 fixation.