Pitting corrosion of pure iron in borate buffer solution was studied by in-situ technics using chloride isotope 36Cl. Relationships between chloride ion concentration and current density in pit during pitting corrosion was discussed. Some results are summarized as follows. (1) Pits growth process is divided into two steps by the variation of chloride ion concentration and pitting current with time. The first step: chloride ion concentration on a pit surface (γp) increases linearly with time while the current density (ip) stays constant. The second step: γp stays at a constant, on the other hand, ip decreases gradually with time. (2) Both of γp and ip increase with external electrode potential. (3) Also γp and ip increase with a logarithmic low of the chloride ion concentration in a bulk solution.