It is desirable to integrate assembly planning with design early in the product development process. Usually, assembly plans can be generated by using either automated or interactive approaches. However, both kinds of approach have limitations. Interactive approaches require extensive user input in the tedious form of answering questions, whereas automated approaches can only be applied to products with simple component configuration and geometry. Based on virtual prototyping (VP), an intuitive and efficient approach is proposed for assembly planning by combining the potential strengths of the two kinds of approach. The main idea of this approach is to extract human expertise of mechanical assembly from human-computer interaction, and then use it for the automatic generation and interactive evaluation of assembly sequences. To incorporate expert knowledge, one or a few initial sequences are firstly created in a teaching by showing way using the human-computer interface of virtual reality, where some intuitive and meaningful information such as motion path and assembly tool is captured automatically. Then, after representing assembly constraints as precedence expressions by reasoning about these initial sequences, all feasible sequences are generated efficiently based mainly on simply symbolic reasoning in place of costly geometric reasoning. In this way, the complexity of assembly sequence planning is reduced dramatically from seeking paths in a high-dimensional state space into a discrete graph-based searching problem. Finally, practical or good sequences are selected from the feasible sequences by exploiting the VP intuitive manipulation capability for judging the cost of a particular assembly operation. In the present authors' experience, VP-based interaction has been proven to be a powerful and intuitive paradigm for both generation and evaluation of assembly plans. The feasibility of the proposed method is demonstrated through an implemented prototypical system called VPASPE, which has been tested with some practical assemblies.