\input zb-basic \input zb-matheduc \iteman{ZMATH 2014d.00592} \itemau{Hong, Gap ju; Song, Myeong seon} \itemti{The relation of the angle sum of a triangle and the property of parallel lines in elementary school mathematics.} \itemso{J. Korean Soc. Math. Educ., Ser. C, Educ. Prim. Sch. Math. 16, No. 2, 183-192 (2013).} \itemab Summary: This study points out that the angle sum of a triangle and the property of parallel lines are taught without showing any relations between them on elementary school mathematics textbooks. This study looks into the structure of Euclid {\it Elements} and it discusses the contents of current Korean textbooks. The property of alternate angles and corresponding angles of parallel lines is inherent in many subjects in elementary school mathematics, and has a meaning that must be taught with the angle sum of triangles in the structure of Euclid {\it Elements}. With this consideration, this study makes a conclusion that these two subjects should be taught by presenting relations between them. \itemrv{~} \itemcc{G40 U20} \itemut{angle sum of a triangle; Euclidean geometry; properties of parallel lines} \itemli{} \end