Improving ideas about nature of science (NOS) is one of the prerequisites for making individuals scientifically literate, which is one of the common aims of the curricula recently developed. However, the studies show that even some science teachers have naïve ideas about NOS. In this qualitative study, a professional development package (PDP) developed by researchers to improve in-service and pre-service science teachers‟ understandings about NOS is introduced and its effects on pre-service science teachers‟ NOS understandings is examined. PDP included explicit-reflective scientific inquiry and argumentation activities and 27 pre-service chemistry teachers trained with the PDP for one year. Variety of qualitative data sources including Views on Nature of Science-Form C, interviews and self-evaluation forms were used in the study. Findings of the study indicated that PDP has improved pre-service teachers‟ ideas about NOS. However, participants‟ misconceptions about the scientific theories and laws, observation and inference have been more difficult to change. Results of the study also showed that changing understandings about NOS requires long and well designed instructions.