Objective: To evaluate the value of magnetic resonance (MR) in the diagnosis of gastrocnemius tendinosis (GT). Methods: Retrospective review was completed on randomly selected 400 cases obtained MR knee exams from October 2014 to September 2015. Axial T2 and sagittal PD images with fat suppression were double-blinded reviewed by 2 radiologists. The gastrocnemius tendon femoral attachments were graded as normal, mild (few cysts, thickening, and intermediate signal) or severe GT (multiple cysts, marrow edema, and tear). The correlation between GT and posterior knee pain was analyzed. Results: Among 350 patients, 179 patients were diagnosed with GT, including 147 cases of mild GT and 32 cases of severe GT. Among these, mild tendinosis was most commonly seen involving the medial head of the gastrocnemius in 105 cases followed by synchronous medial and lateral heads in 31 cases, and just the lateral head in 11 cases respectively. Severe tendinosis was most commonly seen involving synchronous medial and lateral heads of the gastrocnemius in 17 cases followed by the medial head in 11 cases, and just the lateral head in 4 cases respectively. However, patients with severe GT are more likely to present with posterior knee pain than patients with mild GT (76.1% vs 23.9%, P<0.05). Conclusion: MR is a reliable tool for diagnosing GT. GT presents predominant involvement of the medial head, mild in severity, presents frequently as posterior knee pain.