The purpose of this study was to identify primary students’ conceptual understanding and alternative conceptions in acid-base. For this reason, a 15 items two-tier multiple choice test administered 56 eighth grade students in spring semester 2009-2010. Data for this study were collected using a conceptual understanding scale prepared to include the concepts used in the subject of “Acids and Bases”, which is a part of the unit “Structure and Properties of Matter” taught in the eighth grade Science and Technology course. The conceptual understanding scale was developed by the researchers to identify the alternative conceptions students might have concerning this subject. The scale consists of diagnostic tests (n=15) designed to measure levels of understanding among students concerning the subject of acids and bases, and to identify their ways of thinking and rationales. Data were first analyzed by tabulating students’ answers to the first tier of each question, and the percentages of the reasons they selected for their answers. Analysis of results showed that students find difficulty about conceptual understanding and they have some alternative conceptions related to in acid-base.