A study was performed to predict the apple diameter of cvs. Granny Smith and Red Spur in three orchards from San Fernando, VIth Region of Chile. Models for predicting apple diameter at harvest as a function of degree-days were developed for sizes 80, 88, 100, 113 125, 138, 150 and 163, using the logistic growth equation. This equation in average was able to predict the apple diameter with a standar error of estimate of 3.45 mm, an absolute error of 3.1% and a coefficient of correlation of 0.93