A child version of mental accounting tasks has been administrated on 180 3- to 5-year-olds to tentatively investigate the development of young children's mental accounting. The results show that the development of young children's mental accounting varies along with age in the three ways in which the non-fungibility effect of mental accounting takes effect. In the expenditure loss task (ELT), mental accounting develops very slowly, from .03 at age 3 to .08 at age 5; in the income source task (IST), it develops still slowly, from .10 at age 3 to .27 at age 5; in the saving intention task (SIT), it develops much better than that in the above two tasks, from .33 at age 3 to .57 at age 5. There is no difference according to sex in the effect of mental accounting in young children's economic decision-making.