Mentally handicapped highschool students and non-handicapped first graders were given tasks of reading geometry text and multiple choice tests of geometric concepts. The text and test questions were displayed on a computor screen. Here, the text illustrated axis symmetry notions by displaying wings of butterflies and collision of toy cars with their own mirror images. In causal modeling analyses, test remarks obtained for image rotation, working memory capacity and a few geometry notions were used as predictors of comprehension of axis symmetry. The results showed causal relations between working memory capacity and comprehension of symmetry when subjects were given no question aids in their reading tasks. The results were interpreted in terms of construction and management of mental models in working memory of subjects (Glenberg & Langston, 1992).