Objective: This study aimed to identify the prevalence and risk factors of knee pain and radiographic knee OA (RKOA) and to investigate whether decreased lower extremity muscle mass (DLEM) might related with knee pain and also its severity.?Methods: We performed a cross-sectional study using data from 3,278 participants who aged over 50 and took dual x-ray absorptionmetry, plain knee radiographs and knee pain questionnaire in the fifth Korean National Health and Nutrition Examination Survey. Lower extremity muscle mass (LEM) was defined as the sum of the fat-free soft tissue mass of legs, and lower extremity muscle mass index (LMI) was calculated as LEM/body weight (%). DLME was defined as an LMI was below 2 standard deviations from mean in sex-matched young reference group. Categorical variables were presented as number (weighted %) to produce nationally representative data.?Results: The prevalence of knee pain and RKOA were 22% (n＝721) and 34.7% (n＝1,234) respectively. Female (OR 1.5, 59% CI 1.14-1.98), old age (OR 1.02, 95% CI 1.00-1.03), lower education (OR 1.64, 95% CI 1.30-2.08), bed rest (OR 2.14, 95 CI 1.53-3.00), RKOA (OR 1.77, 95% 1.42-2.22), stiffness (OF 12.91, 95% CI 9.52-17.51), and DLEM (Or 1.47, 9% CI 1.02-2.13) were significantly related knee pain after multivariable logistic regression analysis. We found participants with RKOA and DLEM simultaneous had complained more severe pain (pain score 7.18±2.48) than those with participants with knee pain without RKOA and DLEM (5.02±2.44), those with only RKOA (6.29±2.50), and those with DLEM (6.78±2.18) (p＜0.001). These result remained after adjustment for age, sex, education, smoking, vitamin D insufficiency, osteoporosis, treatment history, stiffness, obesity, bed rest, waist circumference.?Conclusions: Prevalence of knee pain and RKOA were high in general Korean population. Decreased lower extremity muscle mass was an independent risk factor for knee pain and it did amplify knee pain severity. This finding supports current guidelines that emphasized the importance of muscle mass and strength to reduce knee pain.