The KSTAR Widget Toolkit (KWT) was developed as a development toolkit of CA client application for the KSTAR commissioning. The KWT is based on Qt library and includes channel access interface to communicate with EPICS. In order to enhance development speed and increase aesthetic quality of application, 16 plug-in widgets were developed to enable for developers to create new panel using drag and drop method. Some of them use QWT as a plotting library and some widgets display alarm status with a specified color according to the EPICS alarm convention. The KWT has cross-platform development environment and feasibility of extending new widgets using Qt plug-in API with plenty of documents and tutorials. Around 110 panels and several applications such as multi-channel plotting tool, process variable searching tool, and logbook application were developed through the KWT and they proved functionality of the KWT being used for the integrated control and machine control during the KSTAR commissioning. The KWT is applicable to fast and easy development of operator interfaces and applications for the EPICS based control system.