Constraint-based partial evaluation (CBPE) is a program optimization technique based on partial evaluation (PE) and constraint solving. Conventional PE only utilizes given parameter values to specialize programs. However, CBPE makes use of not only given values but also the following information: (a) the relationship between input parameters and program variables; (b) logical structure of a program to be evaluated. In this paper, a formal description of CBPE method for imperative languages is presented, and some related problems are discussed.