Due to the rapid evolution of graphics hardware, interactive 3D graphics is becoming popular on desktop personal computers. However, it remains a challenging task for a novice user equipped with a 2D mouse to navigate in an architectural environment efficiently. We think the problem is partly due to the fact that precise navigation control is difficult to achieve with low frame rates. In this paper, we propose a novel approach to improve the effectiveness and efficiency of 3D navigation for architectural walkthrough applications. We adopt a path planner with a probabilistic roadmap to help users avoid unnecessary maneuvers due to collisions with the environment. We modify a Java3D implementation of a VRML browser to incorporate the path planner into the user interface. Experiments show that our implementation of the path planner is very efficient and can be seamlessly incorporated into the navigation control loop. The overall navigation time for traversing a sequence of checkpoints in a maze-like environment can be improved by about a factor of two if the intelligent user interface is used.