An un-interruptible power supply (UPS) with indication of battery internal resistance includes a processor, a display, a sensing module and a battery pack. The sensing module is connected to the processor and the battery pack. The processor is connected to the display, generates a reference internal resistance according to multiple internal resistances detected by the sensing module within a stable time after installation, generates a stable internal resistance according to an internal resistance of the battery pack detected by the sensing module after the stable time, compares the stable internal resistance with the reference internal resistance to generate a status of internal resistance of the battery pack, and instructs the display unit to display the status for user's timely awareness of the condition of the battery pack and prompt action when the performance of the battery pack deteriorates.