Sea turtles are endangered marine-adapted reptiles that are obligate terrestrial nesters. They have a nesting process that involves a series of behavioral phases: (1) crawling, (2) digging a body pit, (3) digging an egg chamber, (4) depositing eggs, (5) covering the egg chamber, and (6) camouflaging the site. Discrimination among these behavioral phases is necessary for estimating which parts of the nesting process are affected by human activities or characteristics of beaches. We calculated multivariate features from tri-axial accelerations obtained by a data logger to extract key features for discriminating between these behavioral phases of green turtles (Chelonia mydas). The developed decision tree classifier discriminated these behaviors with high accuracy, resulting in 5 behaviors after combining digging a body pit and camouflaging the site (86.207 %). The structure of the tree showed key features for discriminating the 5 behaviors on the basis of differences in the movements of turtles. The decision rules in this study will enable us to discriminate behavioral phases quantitatively without observer interference, thereby providing a basis for estimating turtle energy budget and the influence of human activities on nesting behavior. © 2013 Japan Ethological Society and Springer Japan.