The main purpose of this study is to look into a mutual effect of the cooperative learning upon scholarly achievement in teaching the unit of 'section forces of beam' in the mechanics subject of technical high school students. The definite subject of studies is as follows; Is there any significant difference between STAD cooperative learning group and the lectured learning group? If so, this paper will show which learning group is more effective in the post scholarly achievement examination, which group is better in the higher, middle and lower group of study record, and which area is stronger among the area of knowledge, comprehension, application and analysis. The main result of this study is as follows; 1. The data analysis by the t-verification shows that the STAD cooperative learning group got a little bit higher point than lectured learning group in the total score of post scholarly achievement examination, but there was no statistical significant difference. 2. In the lower grade group, the cooperative learning group got a relatively higher point than the lectured learning group in the total score of post scholarly achievement examination, but the data analysis by the t-verification appeared to be no statistical significant difference. 3. In all areas of cognition, the cooperative learning group got a little bit higher point in the score of post examination, but there was no statistical significant difference. In conclusion, as a result of post scholarly achievement examination, however the STAD cooperative learning in the unit of 'section forces of the beam', the mechanics subject of technical high school, showed it to have a little bit higher point only in the side of score, but it is very hard to expect a great effects.