This quasi-experimental study used a repeated measures design to examine the effects of three manipulated classroom goal structures (i.e. mastery, performance-approach, performance-avoidance) on grammar achievement, perceived autonomy support, flow experience, and affect for 49 Greek elementary school students (ages 11-12; 48% girls). Over a period of four weeks, all students participated in three grammar classes, each emphasizing a different goal structure. Results from multilevel modeling showed that students in mastery and performance-approach goal structures achieved higher grammar scores than they did in the performance-avoidance goal structure. Additionally, students in mastery goal structure experienced greater autonomy support, flow, and positive affect than they did in the performance-approach and performance-avoidance structure. The results have implications regarding teachers' training since classroom practices are connected with achievement and students' engagement.