In this study, the lengths of the main links of the transplanting device of a semi-automatic vegetable transplanter were optimized to reduce the weight at the same planting trajectory. The theoretical planting trajectory was obtained from the kinematic analysis for the link structure of the transplanting device and verified through kinematic simulation using commercial software and actual measurement using high-speed camera. Then, the lengths of the main links that have a great influence on the planting trajectory were optimized to have a minimum total length at the same planting trajectory. A genetic algorithm was used as an optimization tool. As a result, with the optimal lengths of the main links, the same planting trajectory was maintained while reducing the total length by 18.32% compared to the conventional one. The transplanting device with the optimal main link lengths would have benefits in terms of agricultural economy by reducing manufacturing and fuel costs.