The purpose of this study was to investigate 8th grade students' reasoning skills on measurement. Specifically, this study aimed to find students' reasoning skills on the case of surface area and volume of a cylinder. The data were collected from 271 8th grade elementary school students enrolled in three public and two private schools in Ankara at the end of 2008 spring semester. In order to investigate the elementary students' reasoning skills regarding the area and volume of a cylinder, the Cylinder Exploration Task (CET, Merseth, 2003) was administered as the measuring instrument. The results revealed that 8th grade students had difficulty in solving the problems demanding the conceptual understanding on reasoning and measuring the surface area and volume of cylinder. Discussion of findings and recommendations for future research studies were provided.