Korean ginseng has been a major cash crop in Korea and the importance of ginseng industry has been widely accepted. However, while there is an increasing demand for 6-year-old ginseng, producing 6-year-old ginseng is not an easy task because of production risk (e.g., diseases, insect pests) involved in the mature process of ginseng. Thus, from a ginseng growers’ perspective, optimal harvest decision is important. This paper uses an asset replacement model to investigate optimal harvest decision. By utilizing a discrete dynamic programming approach, it was found that the productionof 4-year-old ginseng is an optimal choice under the production risk involving gray mold which is one of the major ginseng related diseases. The sensitivity analysis also supports this finding.