This paper describes an empirical study (n=352) into Greek in-service primary school teachers’ conceptions of concepts of force and weight. A closed multiple-choice questionnaire is given to primary school teachers as a tool to explore conceptions, which has been used in previous research. The study population consisted of teachers at primary schools who work in the Greek education directorate. The research data is related to the teachers’ years of service, from where it is studied whether their teaching experience changes/reduces the alternative conceptions. The research data investigated whether the teachers’ teaching experience alters or reduces alternative conceptions based on their years of service. Years of service correlate statistically significantly with most questions. In particular, our results indicate that the alternative conceptions of teachers, reflecting misconceptions and preconceptions, reduce as the years of professional experience increase. Our study can be employed in science teaching, the design of Curricula, and teachers’ professional development.