The present study was carried out to examine the influence of aptitude and metacognitive knowledge on solving arithemtic plane figure task. 51 fifth graders were divided to four groups; LA/LM, LA/HM, HA/LM, HA/HM on the basis of aptitude and metacognitive knowledge. Problem solving process were acessed with Worksheet and Stimulated-recall Interview. The main findings were as follows; (a) high aptitude children had higher performance than low aptitude children in planning and executing stages, (b) HA/HM children evaluated their answer more accurately than the others, and (c) high metacognitive knowledge children showed richer awareness than low metacognitive children in planning stage.