This paper describes Web Page Change Detection System for Selected Zone based on tree comparison mechanism corresponding to HTML pages. Two sub trees for the selected zone will be generated one for initial and another for changed version of Web Document. The Generalized Tree Comparison Algorithm is developed to compare these sub trees for selected zone. This algorithm uses the properties of HTML page and heuristics as a node of the trees. This proposed system will include functionalities and interfaces for processing user request, fetching web pages from the internet allowing users to select zone in web pages to monitor. This method performs well and is able to detect the structural as well as content level changes even at the minute level and helps to locate minor or major changes within the selected zone of document.