This study used fraction number line estimation tasks to evaluate students' developmental patterns, and the relationship of such tasks with whole number bias was explored. In total, 189 fourth-grade students in a northern Taiwan elementary school were followed over 2 years. The results demonstrated that the students' fraction learning development pattern progressed from being nonlinear to linear, and none of the students' estimations exhibited a logarithmic pattern, as was the case in positive-integer number line estimation tasks. Many of the fourth-grade students (41.3%) did not exhibit a linear pattern of estimation and had clear whole number bias. By the fifth grade, half of these students still had a nonlinear pattern and could not treat fractions as integrated numbers. Instead, they were affected by whole number bias and mistakenly used the denominator to judge the actual fraction value.