The purpose of this research was to identify the relationship between task performance and menstrual cycle. The difference of performance on menstrual cycle phase was investigated. The task was the mental arithmetic task which involved the non-sequential and higher order cognitive processes. The duration of the experiment was twenty minutes. Two-way analysis of variance by repeated-measures design was used to examine the differences in task performance between phases and temporal variations. Results showed that there was a significant difference in correct input time during temporal variations though there was no significant difference between phases. Moreover, the relationships between phases and intra-individual variations in task performance were examined using coefficient of variance (CV). CVs were plotted in three dimensions to examine the relationships between intra-individual variations and phases. Based on CVs, the subjects who showed differences were classified into two groups: those with a small difference in three phases and those with a difference every phase. The phase which indicated large CV changed with individuals.