Estimation of the performance of photovoltaic (PV) module and system by using continuous monitoring data is an important issue, since the output power of PV modules is changeable, affected by the irradiance, temperature and shading effect, as well as degradation. This work proposes a method to detect partial shading on a module during its maximum power point tracking (MPPT) operation. It identifies whether there is shading on a module or not by analyzing data of the voltage at maximum power (V-mp) and the current at maximum power (I-mp). Recently developed temperature correction formulas for the Vmp and Imp are used, in order to analyze them under various temperatures and irradiances. The experimental and simulation results show that the shading effect usually results in larger Vmp than the shadeless case, compared at the same Imp. Therefore, the Imp Vmp curve shifts toward higher voltages by the shading effect, thereby enabling detection of the existence of the shading effect. A method to identify the shading effects on a PV module from the I-mp - V-mp curve has been clarified, which does not require I-V curve measurements. Slight partial shading such as a drop in the photocurrent of a single cell in a module by about 5-10% is possibly detected. The present results are also applicable to PV systems that include multiple modules. They are expected to be useful for accurately monitoring the performance of PV modules and systems under operation, since the Vmp and Imp are measurable without breaking the MPPT operation.