Causal reasoning is the core and basis of cognition about the objective world. This experiment studied the development of causal reasoning in 86 3.5-4.5-year-olds using a ramp apparatus with two input holes and two output holes [Frye, D., Zelazo, P. D.,&Palfai, T. (1995). Theory of mind and rulebased reasoning. Cognitive Development 10, 483-527]. Results revealed that: (1) children performed better on cause-effect inferences than on effect-cause inferences; (2) there was an effect of rule complexity such that uni-dimensional causal inferences were easier than bi-dimensional inferences which, in turn, were easier than tri-dimensional causal inferences; and (3) children’s causal reasoning develops rapidly between the ages of age of 3.5 and 4 years.