Two small mountainous sub-watersheds, located at Serikawa river basin on Shikoku Island of Japan were studied for streamwater chemistry. The objective of this study was to determine the impacts of reforestation subsequent to partial logging on streamwater chemistry of the treated sub-watershed compared to the adjacent undisturbed one after several months of reforestation. Both of the sub-watersheds have similar geological and climatic conditions except they vary in size. We collected bulk precipitation and streamwater samples in the pair of sub-watersheds for a period of three years, and analyzed for 10 chemical components (pH, EC, Cl^-, NO₃^-, SO₄2^-, Na⁺, NH₄⁺, K⁺, Ca²⁺, and Mg²⁺). Non-parametric statistical analyses were applied in this study. The pH, EC, SO₄^2-, and cation concentrations in the streamwater were almost unaffected by the reforestation subsequent to partial logging,whereas NO₃^- and Cl^- were sensitive to the forestry activities since their concentrations largely fluctuated in the streamwater of the treated sub-watershed. Results suggested that the increasing NO₃^- and Cl^- concentrations in streamwater caused by the partial logging may be recovered by subsequent reforestation within more or less two years.