This research was conducted in order to investigate the guidance school teachers' stipulated and tacit knowledge about learning theories and applying it in the teaching process. The sample consisted of 266 teachers (146 females and 120 males) who were selected randomly from among Tehran guidance school teachers. The data was collected using two researcher-made questionnaires. The data was analyzed, using descriptive and inferential statistics and the following results were obtained: There was a significant difference (p<.05) between observed average stipulated (.33) and tacit knowledge (3.61), and there was a weak correlation between two kinds of knowledge on learning theories (r=0.144). There was a significant difference between teachers' knowledge on learning theories and variables such as, academic document, gender, and teachers' teaching experience. However, there was not a significant difference between the teachers' knowledge and the length of their in- service training (p≥.05). So, enhancing the teachers' stipulated knowledge of learning theories and special attention to teachers' tacit knowledge should be considered by the educational authorities and planners.