Islanding occurs when a Distributed Generation (DG) source continues to energize an isolated section of a power system even after it was disconnected from the main power grid. Since islanding can cause hazardous conditions to people and equipment, current utility standards require that islanding be quickly detected by protective relays and inverters that are parts of the DG system. Passive islanding detection techniques, unlike their active counterparts, monitor system parameters without injecting any disturbance into the grid. Although widely used, passive detection techniques are not very effective in detecting islanding especially in cases where there is small power mismatch and they also may trigger false detection in some non-islanding cases. To address these drawbacks, a novel and effective passive islanding detection technique that conforms to standard regulations has been presented in this paper. The proposed detection technique is based on monitoring the oscillations in the Rate of Change of Frequency (ROCOF) measured at the Point of Common Coupling (PCC) in the system. The proposed detection technique was developed and tested on a grid connected photovoltaic DG system using simulation. Results indicated that this technique was not only capable of detecting islanding when it occurs but also able to accurately distinguish between islanding and non-islanding under a wide range of operating conditions.