There are growing data and a continuing controversy over the effect of folic acid supplementation on cancer risk. We conducted a meta-analysis based on up-to-date published relevant randomized trials to further examine this issue. Relative risk (RR) was used to measure the effect of folic acid supplementation on risk of cancer using a random-effects model. Overall, folic acid supplementation had no significant effect on total cancer incidence (13 trials,n= 49,406, RR = 1.05; 95% CI: 0.99–1.11,p= 0.13), colorectal cancer (seven trials,n= 33,824, 1.01; 0.82–1.23,p= 0.95), other gastrointestinal cancer (two trials,n= 20,228, 1.00; 0.75–1.33,p= 0.99), prostate cancer (five trials,n= 27,065, 1.17; 0.84–1.62,p= 0.35), other genitourinary cancer (two trials,n= 20,228, 0.97; 0.75–1.27,p= 0.84), lung cancer (five trials,n= 31,864, 1.00; 0.84–1.21,p= 0.97), breast cancer (four trials,n= 19,800, 0.82; 0.63–1.07,p= 0.15), hematological malignancy (three trials,n= 25,670, 0.87; 0.64–1.17,p= 0.35) and total cancer mortality (six trials,n= 31,930, 1.02; 0.90–1.15,p= 0.81). However, a significantly reduced risk was observed for melanoma (three trials,n= 19,128, 0.47; 0.23–0.94,p= 0.03). Furthermore, higher total cancer incidence risk was observed among those trials with a higher percent use of lipid-lowering drugs (>60%, 1.10; 1.00–1.20,p= 0.04), or with lower percent baseline hypertension (≤70%, 1.08; 1.00–1.16,p= 0.057).Consistently, meta-regression analyses suggested that the similar trend between percent use of lipid-lowering drugs (p= 0.084) or percent baseline hypertension (p= 0.056) and log-RR for total cancer incidence associated with folic acid supplementation. Our findings indicate that folic acid supplementation has no significant effect on total cancer incidence, colorectal cancer, prostate cancer, lung cancer, breast cancer or hematological malignancy, but reduces the risk of melanoma.