Muscle strength, an independent predictor of metabolic disorders, disability, and mortality, reduces gradually with advancing age. Little is known about the influence of nutritional intervention on muscle strength in middle-aged. The aim of the present study is to examine whether magnesium could improve body composition and muscle strength in middle-aged overweight women. In this double-blinded, placebo-controlled, randomized trial, a total of 74 healthy middle-aged overweight women (25 ≤ BMI ≤ 30 kg/m(2)) received either 250 mg magnesium in the form of magnesium oxide or placebo daily for 8 weeks. Body composition was assessed using Bioelectrical Impedance Analysis (BIA). Handgrip strength and knee extension strength were measured with isometric dynamometry. Functional mobility was assessed using Time Get Up and Go Test (TGUG). A significant increase in mean lean body mass was observed (P = 0.05) accompanied with a significant decrease in fat mass (P = 0.02) solely in the magnesium group at the end of 8 weeks compared to baseline values but the changes did not reach significant as compared to placebo group. Handgrip strength and TGUG improved in the magnesium group compared to baseline but they were not significant compared to placebo. There were no significant differences in increasing knee extension strength in the magnesium group as compared with placebo. Baseline values of serum magnesium and muscle strength of participants did not indicate any influences on response to magnesium supplementation. Our findings indicate that magnesium as magnesium oxide, 250 mg/day, for 8 weeks do not lead to a significant greater gain in muscle strength and function compared to placebo.