This study is aimed to describe the increase of the mentally retarded students vocabulary mastery by using Scaffolding method with seventh the graders of SLB N Jepara in the academic year of 2014/2015 and to know how effective the implementation of using scaffolding method to increase the mentally retarded students vocabulary mastery is. The design of this study is classroom action research that was conducted in three steps (pre-cycle, cycle I and Cycle II) with four activities in cycle I and cycle II, they are planning, acting, observing, reflecting.
The result of this study showed that using Scaffolding method can increase the mentally retarded students vocabulary mastery. This is proved by students skill test that increased in every cycle. In the pre cycle, the average of students score was 53, it meant fair. And the observation result of students activeness was 48%, it also meant fair. In the first cycle, the average of students test got 73, whereas the observation of students activeness score got 72%. Based on the result of first cycle, it showed good. And in the second cycle students test got 87, and the observation of students activeness score got 76%, it meant excellent. Teaching learning process ran well. There were some significant improvement from pre cycle, first cycle, and second cycle. There was increase in every cycle after using Scaffolding method. Thus, it can be concluded that the used of Scaffolding method in teaching vocabulary mastery was increase and effective for mentally retarded students.