This research is about using two different instruction models, "theory course combined with sample introduction" and "theory course combined with case method teaching", to instruct elementary teachers on how to guide the science fair product in two courses (16 and 12 teachers in each class) and observe their guiding tactics after the instructed classes. The results show that: (1) Elementary teachers who have taken "theory course combined with sample introduction" course consider that: (a) Introducing the samples can let them clearly understand the process of how to guide students to do their science fair project; and (b) Following the description sample to make their science fair project topic, extend these topic form original science courses, draw the conception map and flow table, handle the scientific experiment, and then teach students to be familiar with the content of science fairs project; (2) In-service teachers who have chosen "theory course combined with case method teaching" course consider that: (a) Case-method teaching helps them understand the contents of the curricula; and (b) It provides them models to observe and imitate. With such an increase of awareness, knowledge transference had been brought out. Thus, professional knowledge would be promoted. Both teachers who have accepted these courses had hiatus when guiding students to develop their product: (1) Teachers are inadequately comprehending the basic scientific theory of subjects of their science fair project; (2) Scientific verification is not scientificalness; (3) Verify facts which are already known; and (4) Be careless about the control variable. (Contains 3 tables.)