An adequate understanding of the nature of science (NOS) is essential for science teachers. The Myths of Science Questionnaire (MOSQ) consisting of 14 items, which comprised both optional and written types of response, was utilized to explore 113 Thai preservice and 101 inservice science teachers' understanding and reasoning about the NOS, particularly scientific knowledge, scientific method, scientists' work and scientific enterprise. A highly consistent pattern of responses emerged regarding the status of theories and laws, a hierarchical relationship between theory and law, tentativeness of science and science as cumulative knowledge. Most of them were uninformed understanding of the NOS. The MOSQ expressed, to some extent, the effectiveness in illustrating respondents' reasoning about the NOS. The implications concerning the MOSQ and science teacher preparation and professional development are also discussed. (Contains 6 tables and 1 figure.)