Few studies have investigated the effects of the instructions provided in educational computer games on cognitive processing and learning outcomes. In our experiment, we sought to compare the effects on learning outcomes of two different types of goal-oriented instructions: "mastery-goal" instructions, which prompt learners to develop skills or master new knowledge, and "performance-goal" instructions, which are frequently used in game environments and which encourage individuals to demonstrate their ability to succeed, particularly by surpassing others. Results showed that a mastery-goal instruction elicited deeper learning (as assessed with a transfer task) than a performance-goal instruction. No effect of instruction was observed on either learning (demonstration consultation) times at the start of the game or on training task (solving riddles) performances during it. These results are discussed in terms of learning processes. This study demonstrates that mastery goal-oriented instructions can promote active processing of educational content in a serious game environment.