The purpose of this study was investigated the onset time of rectus abdominalis, deltoid and trapezius muscle contraction during sit to standing(STS) positions between health children and children with spastic diplegia. This study group was consists of 15 children with spastic diplegia who were selected from the outpatients at the Severance Hospital and control group with age matured 15 normal children who were chosen at an elementary school nearby the Severance Hospital. The superficial electomyography was applied to the three muscle groups(rectus abdominalis muscle, deltoid and trapezius muscles). Onset time of the contraction in three muscles group of the upper part during STS position were recorded automatically by a computer. The major findings of this study were as follows : (1) The onset time of three muscles contraction at the upper part of a body during STS positions were found to be significantly different between the children groups of normal and spastic diplegia: those were longer for the children with spastic diplegia(p<0.05). (2) Each of the right and left muscle onset time of three muscles contraction of right and left side of upper part of body during STS was not significantly different between normal children and children with spastic diplegia. Further research is need to clarify the coordination and simultaneous contraction issues between agonists and antagonists during STS positions between health children and the children with spastic diplegia.