The aim of this study is to develop an Astronomy Concept Inventory (ACI) to assess undergraduate students' conceptual understanding of moon phases, moon and earth motion. ACI consists of 25 multiple choice questions with four distracters. The inventory was administered to 262 undergraduate students who took introduction to astronomy course. The data were analyzed by ITEMAN software program. The difficulty indices ranged from .45 to .92, and discrimination indices ranged from .21 to .59. The mean P value as difficulty index was found to be .66 for the test while alpha value for whole test was found as .80. (C) 2011 Published by Elsevier Ltd.