A study was conducted to analyze fluctuations of physical and mental condition during sports training using a single-case study design and statistical techniques. Two male youth soccer players aged 18 years, one participating in a national high school sports competition(sub.A), and the other in a prefectural high school sports competition(sub.B), independently recorded their physical and mental condition and lifestyle on the new version of the "Quality Control(QC)sheet" every day for 70 days(sub.A)and 120 days(sub.B). The 12 items related to lifestyle, and physical and mental condition were analyzed statistically. Factor analysis with principal component solution and promax rotation was utilized to extract components related to fluctuations in condition. Randomization test and interrupted time series analysis(ITSA)were used to compare means, and the slope of the regression equation between sequential training periods. Five fluctuation factors-evening living time, morning living time, physical condition, physical and mental fatigue-were extracted, and these explained 80% of the total variance for sub.A. For sub.B, five fluctuation factors-morning living time, evening living time, condition, fatigue and body weight-explained 84% of the total variance. Significant differences in means of the factor score for physical condition fluctuation were found between in-and postseason of the national competition for sub.A, and between the 4 sequential training periods for sub.B(P<.05). These results indicate that the physical condition of youth soccer players shows significant individual fluctuations between sequential training periods in the pre-, in-, and post seasons. It was concluded that fluctuations of physical and mental condition can be confirmed statistically using a single-case study design and statistical techniques such as randomization test and interrupted time series analysis(ITSA).