The authors compared the mental computation performance and mental strategies used by an experimental Grade 2 class and a control Grade 2 class before and after instructional intervention. Results indicate that students in the experimental group had better performance on mental computation. The use of mental strategies (counting, separation, aggregation, and holistic) and atypical strategies in the control group decreased or even disappeared after the intervention. However, students in the experimental group could apply higher level mental strategies, such as aggregation, holistic, short-jump, and atypical strategies after the intervention. These findings indicate that the teaching of the vertical addition/subtraction algorithm does not promote the development of mental computation.