The nitriding process is one of the common methods for surface hardening, and consists of heat treatment in a furnace for many hours. The nitriding behavior and strengthening mechanism of Ti added steels in the nitriding process, which is applicable to a high temperature and rapid process such as the continuous annealing of steel strip, were investigated. The sheets were hardened only near the surface. The hardening of the surface layer is due to the formation of clusters or fine precipitates with disc-like shape consisting of titanium and nitrogen. The maximum hardness depends on the content of Ti in the steel while the annealing time and the concentration of NH3 influence the depth of the hardened zone affected by nitriding. It is thought that the hardening only near surface improve bending stiffness without significant increase of yield stress. So there is a possibility that the surface hardening improves the dent resistance of automobile outer panels without significant worsening of the surface deflection. To study these behaviors theoretically, a model for predicting the precipitation behavior due to nitriding has been developed. The experimental results can be reasonably explained by the model calculations. And also, the estimation of the amount of strengthening was carried out. It indicated that the strengthening mechanism is mainly the precipitation hardening of TiN that could be Ti nitrides or Ti-N clusters.