It is intractable to deal with dynamic environments, which are totally or partially unknown or even dynamically changing. Several algorithms have been proposed and among such algorithms, potential fields are the core of a class of effective navigation schemes for autonomous robots. This paper proposes an adaptive algorithm for a mobile robot path planning in a dynamic environment where the target and obstacles are moving. This algorithm relies on the integration between a dynamic virtual obstacle concept and a robot size factor, which represents a threat factor to any possible collisions and enables the robot to plan its motion not only with right positions, but also with reasonable velocities. The convergence of this proposal is discussed to guarantee its reliability.