Wearable energy harvester requires a maximum power output with small weight and size to minimize human metabolic cost. Human`s negative work is suitable for driving of the energy harvester because metabolic energy consumption efficiency is higher than normal human work. This study proposes a design of energy harvester based on knee joint`s negative work motion. When a human`s knee performs negative work, a knee`s mechanical power is transmitted to an electric generator through a transmission in the energy harvester. For maximum electrical power output under limited conditions, a gear train transmission is utilized to transfer and amplify mechanical power into generator effectively. This study analyzes selection of electric generator and design of gear train transmission focused on power efficiency and structural stability.