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Systematic works on the effect of grain size on the corrosion resistance of low carbon steel are scant. In the present work, a spectrum of grain sizes was obtained by simple heat treatment routes in a typical low-carbon steel. It was revealed that two distinct stages for the dependency of corrosion current density ( i _corr ) on the grain size exist. Above a limiting average grain size of ∼22 μ m, i _corr decreased slowly with increasing grain size. However, below this limiting value, i _corr increased rapidly, which was related to the increased density of grain boundaries as interpreted by theoretical calculation of number of grains per unit area. Conclusively, a grain size of ∼22 μ m (ASTM grain-size number of 8) was considered to be an optimum value according to the mechanical and corrosion standpoints.