Nowadays, the measurement of micro-milling forces is mainly achieved by a force transducer. However, the frequency of force signal is high, due to the spindle super-speed, which leads to failure of the micro-milling forces measurement by using common force sensors. Additionally, micro force sensors with high-resolution and high sampling frequency are preferred, but they are often expensive. To determine the average micro-milling force with low cost and high precision, we propose an indirect method, by determining the power of the main transmission system of a micro-milling machine. First, the measurement system for the micro-milling machine tool power was introduced, and various sensors were used to measure the current and voltage respectively. Then, a high-frequency sampling system based on the Labview was developed to process the current and voltage signals, and to obtain the power data of the main transmission system. Through this process, the indirect measurement of micro-milling forces was achieved. Finally, we validated the effectiveness of the developed on-line measurement system and the proposed indirect measurement method for average micro-milling force by using experiments. The proposed method is practical and low-cost, and it can lay the foundation for further research on cutting energy consumption.