It has been shown that many of physiological systems have nonlinear dynamics. The evidences of these nonlinear behaviors make us analyze physiological systems in the new viewpoint. And, some of these nonlinear dynamics can be represented by chaotic behaviors, which is studied by several methods-correlation dimension, return map, power spectrum analysis, etc. This study is on the analysis of nonlinear characteristics of ECG. After data have been acquired from 20 children (10-13 years old), and 30 students (20-24 years old). We have calculated parameters HR, PR, VAT, TD, TRD, TPD from data, and estimated correlation dimension, return map, power spectrum, time series. Results show the nonlinear and chaotic characteristics of ECG.