Purposes of this study were to identify the features of implementing operational curriculum in the mixed-age classroom, and discover the factors which affect the preschool teacher conducting this curriculum. The participant was a preschool teacher with 4-5 years old children. The researcher utilized observations, interviews, and document analyses to collect data. The findings of this study are as follows: (1) the participant implements operational curriculum based upon 5 years old children's idea and children's manipulative experiences; and (2) the influence of conducting this curriculum are children's age and the teacher's professional knowledge.