This article presents an experimental study of the primary break-up of a viscous liquid jet interacting with a cross airflow. This experiment was carried out in an open circuit wind tunnel. The measurement test section was made of clear acrylic resin to allow optical access for visualizing and imaging the jet break-up in the cross airflow. The working liquid used in the present experiment was the aero-engine lubrication oil, which was injected at 90degrees into the airflow, via a nozzle from the top wall of the test-section. The effects of liquid viscosity, and the jet and cross airflow velocities on the primary break-up mechanisms of the jet were studied. The main results showed that different break-up regimes were identified; column break-up and bag break-up separated by a transition zone, and new correlations have been proposed for predicting the jet transverse and streamwise penetrations before break-up.