In recent years, VNC (virtual network computing) has become one of the popular tools for a multi-platform application suite allowing users to access graphic displays remotely. However, typical video encoders targeted for high compression efficiency, are inappropriate for mobile VNC systems where computational complexity is of primary concern. In this work, we propose fast modified region detection methods to reduce encoder computation in a mobile VNC server. By encoding and transmitting modified regions only between current and previous screen images, the encoding time can be significantly reduced. We implemented a prototype mobile VNC system, and the proposed methods are practically evaluated. The proposed methods enable fast encoding of screen images by detecting modified regions quickly, and consequently lead to the decrease of encoding time and the increase of screen update rate at a VNC client.