Hi Xie Zhijie,

RadControls are built on top of Telerik Presentation Framework creating complex hierarchies of visual elements and providing property setting inheritance. So, depending on the theme used and its settings, such cases may appear. This is why, it is recommended to introduce the deisred property settings to the respective element. In your case you can set it to the ButtonElement:
            radButton1.ButtonElement.Font = new Font("Segoe UI", 22);