This paper describes a design method targeted to mixed standard- and custom-cells which is carried out as follows. First, it generates a standard-cell design to obtain a maximal achievable speed. Second, it performs a cell customization process to determine which standard cells need to be customized in order to satisfy the given timing constraint. Once designers select a set of standard cells to be customized, they define the specification of the custom cells and develop the cells accordingly. Finally, the designers replace the standard cells with the custom ones and re-evaluate the speed of the design. The customization process continues until the timing constraint is satisfied. In this paper, we present a custom-cell identification method for high-speed datapath-oriented designs. The proposed method uses an HDL-based standard-cell design flow by integrating a custom-cell identification algorithm to determine a minimal-cost cell set and the design budgets for cell customization. Experimental results on a set of benchmarking designs are reported to demonstrate the effectiveness of the proposed method.