Poor plans of design project are often caused by the difficulties in realizing the underlying complexities and immeasurable characteristics in the deisgn process. To overcome this diffculty, a metric approach based on the tasks analysis model of task importance degree was proposed. The model suggests that importance of a task can be measured through the influence to the project incurred by the imaginary failure of the task, which can be measured through three parts: the critical degree of a task to the whole project, the direct cost by the failure of a task, and the indirect cost on the other tasks affected by the failure of the task. The Analytic Hierarchy Process approach was applied to determine the critical degree and Case-based Reasoning idea was used in detecting the implied error in the task. As an application case, a review work planning was given and some conclusions were arrived at.