Radiation pneumonitis is the most frequent acute pulmonary toxicity following stereotactic body radiation therapy for lung cancer. Here, we investigate clinical and dosimetric factors associated with symptomatic radiation pneumonitis in patients with stage I non–small cell lung cancer treated with stereotactic body radiation therapy. A total of 67 patients with stage I non–small cell lung cancer who received stereotactic body radiation therapy at our institution were enrolled, and their clinicopathological parameters and dosimetric parameters were recorded and analyzed. The median follow-up period was 26.4 months (range: 7-48 months). In univariate analysis, tumor size (P = .041), mean lung dose (P = .028), V2.5 (P = .024), V5 (P = .014), V10 (P = .004), V20 (P = .024), V30 (P = .020), V40 (P = .040), and V50 (P = 0.040) were associated with symptomatic radiation pneumonitis. In multivariable logistic regression analysis, V10 (P = .049) was significantly associated with symptomatic radiation pneumonitis. In conclusion, this study found that tumor size, mean lung dose, and V2.5 to V50 were risk factors markedly associated with symptomatic radiation pneumonitis. Our data suggested that lung V10 was the most significant factor, and optimizing lung V10 may reduce the risk of symptomatic radiation pneumonitis. For both central and peripheral stage I lung cancer, rate of radiation pneumonitis ≥grade 2 was low after stereotactic body radiation therapy with appropriate fraction dose.