Children of three to six years of age were presented ten pairs of squares and rectangles differing in area ratios and asked to select a bigger one in each pair. Children's basis of selection was identified as area, height, or width, according to selection patterns of matched two pairs. Few children used width as a basis of selection. Three-and six-year-olds used area more often than four- and five-year-olds who used height more often than three- and six-year-olds. These results were discussed in relation to cognitive development and methodological problems in previous studies.