The reaction of NO+ with water molecules plays a crucial role in the D-region of the atmosphere because the reaction provides nitrous acid (HONO) and protonated water species (H3O+). In this study, the reaction of NO+ with water clusters, NO+ + (H2O)n (n = 1-7), was investigated by means of the direct ab initio molecular dynamics method to elucidate the reaction mechanism of NO+ in the atmosphere from a theoretical viewpoint. At n = 1 and 2, the reaction of NO+ with (H2O)n led to the formation of a complex: NO+ + (H2O)n → NO+(H2O)n (n = 1 and 2). At n = 3, the formation channel of HONO was open, and HONO was formed according to NO+ + (H2O)n → HONO---H+(H2O)n-1 (n = 3), through which H3O+ was also formed as H+(H2O)2. However, the HONO formation efficiency was significantly low for n = 3. In large clusters with n = 4-7, the HONO formation channel became the main channel, and the dissociation of HONO from the HONO--H+(H2O)n-1 complex occurred in part: NO+ + (H2O)n → HONO---H+(H2O)n-1 → HONO + H+(H2O)n-1. The energetics and reaction mechanism were discussed on the basis of theoretical results.