Children's ability to place fractions on a number line strongly correlates with math achievement. But does the number line play a causal role in fraction learning or does it simply index more advanced fraction knowledge? The number line may be a particularly effective representation for fraction learning because its properties align with the desired mental representation and take advantage of preexisting spatial-numeric biases. Using a pretest-training-posttest design, we examined second and third graders' fraction learning in 3 conditions: number line training, area model training, and a non-numerical control. Children who received number line training improved at representing fractions with number lines, and children who received area model training improved at representing fractions with area models. Critically, only the number line training led to transfer to an untrained fraction magnitude comparison task. We conclude that the number line plays a causal role in children's fraction magnitude understanding, and is more beneficial than the widely used area model.