Purpose: The purpose of this study was to develop and validate of a knowledge scale for lymphedema in patients with breast cancer. Methods: 34 preliminary items were made according to literature review, then verified content validity, construct validity, and reliability of the scale. 28 items among them were confirmed through content analysis by 4 experts. After a preliminary test, a survey for 156 breast cancer patients was performed for confirming construct validity and reliability. The data were analyzed using factor analysis, independent t-test, and KR-20. Results: This scale had 28 items consisting of 2 categories: prevention and self-care of lymphedema. Construct validity was confirmed by known-group technique because there were some categories consisted of just one or two items, and some mismatches between categories and items in factor analysis. Women who were educated about lymphedema scored significantly higher than women who had not (t=-3.92, p<.001). Reliability was appropriate (KR-20=.81). A percentage of correct answers was 74.6%, but it was from 31.8% to 96.1%. Conclusion: The study shows that this scale is reliable and valid to measure the knowledge of lymphedema. This scale can be effective to assess and educate the patients with breast cancer.