Background: The postoperative severity of symptoms among women with breast cancer affects their quality of life (QoL). Although it is recommended that performing shoulder-arm exercise 30 min/day can alleviate symptoms and improve the QoL, there is little research on the mediating effects of performing shoulder-arm exercise 30 min/day on the postoperative severity of symptoms and QoL among patients with breast cancer. Methods: A cross-sectional study was conducted 2 ~ 4 months after surgery on women diagnosed with breast cancer but with no distant metastasis and who had undergone breast cancer surgery for the first time. A structured questionnaire was employed which included a severity of symptoms scale, performing shoulder-arm exercise for 30 min/day, a QoL scale, demographic characteristics, and medical status. Results: In total, 117 women with breast cancer completed the survey. The severity of symptoms and performing shoulder-arm exercise 30 min/day separately affected the QoL (B = -0.447, standard error (SE) = 0.050, p \u3c 0.001; B = 15.666, SE = 4.542, p = 0.001, respectively). In model 3, performing shoulder-arm exercise for 30 min/day played a partial mediating role in the relationship of the severity of symptoms and QoL (R 2 = 0.51, F = 5.41, p \u3c 0.001). Conclusions: During 2 ~ 4 months after surgery, regular shoulder-arm exercise for 30 min/day could decrease the effect of the severity of symptoms on the QoL among women with breast cancer. Clinical healthcare providers may inform and educate patients as to the benefits of regular shoulder-arm exercise for 30 min/day