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, and answering its questions, astronomical conceptual test was constructed by the researcher. 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: - The Pre-service science teachers performance in the total astronomical concepts test and in the six domains were below the educational accepted criterion (80%). 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 were 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.