- The aim of this study is to investigate the understanding level of pre-service science teachers at Sultan Qaboos University about astronomical concepts and their alternative conceptions. The study sample consisted of 63 science-major students (male/female) from the college of Education at Sultan Qaboos University. The study was carried out during the academic year 2006/2007. - For the purposes of the study, an astronomical conceptual test was constructed by the researchers. It consists of (18) items spread between six concepts namely: moon phases, the season, tides, solar and lunar eclipses, Sun-Earth-Moon system and planets of the Solar System. The validity of the test was checked out by a panel of 18 judges. The reliability of the test was calculated using Cronbach-Alfa and found to be (0.71) which indicates its suitability for the study purposes. - Results of this study revealed the following: - Although (83٪) of the pre-service science teacher correctly justified that the moon gravity is the reason of the tides, most pre-service science teachers could not present scientific explanation for astronomical phenomena, and only 25٪ of the subjects gave correct scientific explanation. - Regarding the Sun-Earth-Moon model, five alternative models emerged which may explain the shortage in understanding several astronomical phenomena such as moon phases and eclipses. - On the bases of these findings, the study recommends organizing training workshops in order to raise the knowledge level of pre-service science teacher on astronomy. The study also recommends performing additional studies about alternative concepts in astronomy among students and science teacher.