OBJECTIVE To determine the scaling law and design guidelines of small-scale hydraulic systems whose output power is in the range of 10 to 100 Watts. METHODS Fundamental fluid mechanics equations were employed to model the friction and leakage losses in the hydraulic components including cylinders, hoses, and pumps. Basic structural design equations were deployed to predict their weight. Customized test stands were built to validate the efficiency models, and catalog data of o-the-shelf components was compiled to validate the weight models. The electro-mechanical components including electric motors, gear heads and batteries were modeled using their catalog data. RESULTS The efficiency and the weight of both hydraulic and electro-mechanical components were modeled in analytical forms. These models were validated against either experimental data or existing catalog data. CONCLUSION The analytical models suggested the following design guidelines: first, high operating pressure is needed for hydraulic actuation systems to weigh lighter than equivalent electro-mechanical systems; second, critical dimension thresholds exist for hydraulic and electro-mechanical components, which should not be exceeded to achieve reasonable system efficiency; third, component efficiency plays a more important role than component weight to gain higher system power density; lastly, for applications where the actuator system weight matters the most, high pressure small scale hydraulic systems are preferred over electro-mechanical systems, but for applications where the overall system weight matters the most, electro-mechanical systems work better.