In this paper, we present a new sketch-based system - KnitSketch, to improve the efficiency of process planning for knitting garments at an early design stage. The KnitSketch system utilizes sketching interface with the pen-paper metaphor and users only need to draw outlines of different parts of the garment. Based on sketching understanding, the system automatically makes reasonable geometric inferences about the process-planning data of the garment. The system is designed for nonprofessional users and can design diverse garment styles by freehand drawings. The contributions of this work include contextual extraction of reusable data from sketches, a MDG structure for sketch beautification, and an integrated system with natural expression and effective communication that reduces the cognitive load of human beings. User experience shows that the proposed system helps designers focus on the task instead of the designing tools, and thus improves the efficiency and productivity of human beings.