The present study aimed to identify the stage of thinking of practicum students at Najran University in light of Piaget's theory and its relationship to their academic achievement in the scientific disciplines they were studying at the college of science, mainly mathematics, physics and chemistry disciplines. The sample consisted of (50) female student teachers who were practicing teaching mathematics, physics and chemistry at the public schools in Najran. Piaget test was used as a main instrument to determine participants' thinking stage. Results showed that only 10% of participants were in the stage of abstract thinking, 46% were in the transitional stage and 44% were in the stage of concrete operations. Results also revealed statistically significant differences ([alpha]=0.05) between the academic cumulative averages of participants in the concrete thinking and transitional thinking stages in favor of participants in the transitional stage. Moreover, results indicated that there were significant differences between the cumulative academic averages of participants in the concrete and abstract thinking stages in favor of participants in the abstract thinking stage. Nevertheless, there were no statistically significant differences between the cumulative averages of participant students in both transitional and abstract stages. Significant differences were revealed between mathematics and physics student teachers in favor of participants of mathematics discipline. On the opposite, no significant differences were noticed between mathematics and chemistry student teachers, on one hand, and between physics and chemistry student teachers. In addition, there was no significant effect for the interaction between participants' stage of thinking and cumulative average.