In this paper, a new method for the online estimation of equivalent series resistances (ESR) of the DC-link capacitors ininduction machine (IM) drive systems with a front-end diode rectifier is proposed, where the ESR estimation is conducted duringthe regenerative operating mode of the induction machine. In the first place, a regulated AC current component is injected intothe q-axis current component of the induction machine, which induces the current and voltage ripple components in the DC-link. By processing these AC signals through digital filters, the ESR can be estimated by a recursive least squares (RLS) algorithm. Toacquire the AC voltage across the ESR, the DC-link voltage needs to be measured at a double sampling frequency. In addition,the ESR current is simply reconstructed from the stator currents and switching states of the inverter. Experimental results haveshown that the estimation error of the ESR is about 1.2%, which is quite acceptable for condition monitoring of the capacitor.