A precise overload protection is able to improve the thermal design of power supply systems. However, as for LLC resonant off-line converters, there are not many studies about how to detect the load conditions at the primary side of the converters which is important to realize the precise overload protection. In this paper, we implement a lossless and precise load detection technique by a novel average input current sensing method to achieve the precise overload protection. Detailed theoretical analysis is presented with statistical analysis on overload conditions. To verify the analysis, an LLC resonant off-line converter with 400V input and 16V/10A output is simulated using PSIM and a prototype board is built. The theoretical and experimental results show that the precise overload protection is realized based on the novel load detection technique. As a result, the thermal design of the LLC resonant off-line converters is improved.