Abstract: A periodic component is one of the time-varying components in an inconsistent hydrologic series, and quantifying and grading its significance level is practically significant. This paper presents a new method that uses the correlation coefficient to grade the significance level of periodicity in a hydrologic series containing only one simple periodic component. It calculates the correlation coefficient between the series and its periodic component, then uses four selected thresholds of this index to grade the corresponding periodicity significance into five levels: no, weak, mid, strong, and very strong. We derive a relationship of correlation coefficient versus half amplitude of periodicity demonstrating the reasonability of the index used in this method, and examine the influence of each variable on the correlation coefficient. Application to synthetic series and observed hydrologic series shows that our method is reliable and effective, thus helping quantify environmental change and its impact on hydrologic variability.