This study investigated the effect of applying “participant modeling” followed by a role play on grade eleven scientific students’ achievement and motivation in biology. Students usually face problems in understanding difficult biology concepts like Krebs cycle which is composed of a series of biochemical reactions with many molecules and compounds involved. The researcher designed t-shirts with information about the molecules and the reactions of Krebs cycle and students were asked to wear them throughout the teaching/learning process. Implementing participant modeling helped students to memorize the names of the molecules, their role. Then a role play was applied after modeling to relate the chemical reactions together. Nineteen students with ages ranging between 16 and 17 (N=19) enrolled in grade eleven scientific participated in this study. Data was collected using pre and post-achievement tests, and a motivation test that was implemented before and after the intervention. Results showed a significant enhancement in achievement, but no significant change occurred at the level of motivation.  Ultimately, recommendations related to both methodology and future researches are offered.