Valve stiction is the most common cause of the problem on control loops in process industry. To improve the productivity and quality, industrial enbineers demand a tool to facilitate loop monitoring. This paper present a new algorithm to detect valve stiction for diagnosis of oscillation in control loops. The method is for the level control loops and based on a statistical analysis in phase plane by using controller output and level signals, those are available for all the level control loops. The usefulness of the method are successfully demonstrated on a simulation data set and several industrial data sets.