Highly precise outdoor current-voltage (I-V) curve measurement of crystalline silicon photovoltaic (PV) modules has been achieved, by monitoring the solar irradiance with a PV module irradiance sensor (PVMS), measuring the curves in 0.2-0.5 s, and filtering out the data affected by spatial nonuniformity of irradiance. The I-V curves were corrected for temperature by using a recently developed translation formula. Good reproducibility in the short circuit current I-SC, open circuit voltage V-OC, and maximum power P-max measurements was confirmed, based on repeated measurements for one week. Small values of relative standard deviation sigma, i.e., 0.1-0.23% and 0.24-0.42% for the I-SC/G(PVMS) and P-max/G(PVMS), respectively, were confirmed even on partially sunny days and cloudy days when the solar irradiance is very unstable. Here, GPVMS denotes the irradiance measured by using a PVMS. The values of sigma for various kinds of commercial crystalline silicon modules, including the p-type BSF, p type PERC, n-type backside contact and silicon heterojunction, were also investigated in longer periods including different seasons of the year, which were 0.1-0.5% and 0.5-1.3% in the irradiance ranges of 0.9-1.1 kW/m(2) and 0.3-0.5 kW/m(2), respectively. The results have demonstrated that precise outdoor I-V curve measurements are possible in wide range of irradiance under various weather conditions. They are useful for improving the throughput and accuracy of onsite I-V measurements in operation and maintenance. Possible sources of residual measurement error and methods to further improve the precision are also discussed.