Aims: Oral iron traditionally has been administered to patients with chronic kidney disease (CKD). However, there are limited data on the effect of oral iron in CKD patients. Here, we evaluate the effects of oral iron therapy on renal anemia and progression of renal disease in CKD patients. Methods: Anemic patients with nondialytic CKD who were naive to erythropoiesis-stimulating agents (ESAs) were recruited for the prospective observational study. The participants were classified into oral iron or control group, and they were asked to keep their treatment status for 1 year. The primary outcomes were change in Hb and estimated glomerular filtration rate (eGFR). Results: A total of 182 participants were enrolled and 138 completed a 12-month follow-up. No change in Hb level was observed during the follow-up period in the iron group, whereas a significant decrease in Hb was observed in the control group. Oral iron supplementation was effective, especially in patients with eGFR < 30 ml/min/1.73 m2. The changes in eGFR did not differ between the two groups. The incidences of drug-related adverse events were equivalent in two groups. Conclusions: Oral iron supplementation might attenuate the progression of anemia in nondialytic CKD patients without ESAs and not impact kidney function.