The dynamic on-line monitoring system (DMS) based on on-line training of artificial neural network (ANN) has been successfully implemented in a refinery for real-time, on-line and dynamic estimation of quality indexes of fractionator side draw. To improve the predicted accuracy of DMS and fit the new production cases, a novel ANN training method, on-line training method, has been proposed. It can save a lot of time and effort in contrast to the conventional off-line training for ANN. It also improves the reliability of DMS.