A quantitative study on the kinetics of nucleation of primary Si particles (PSPs), especially under the effect of P inoculation, during isothermal melt solidification of hypereutectic Al-Si(-Cu) alloys has been realized for the first time by using a unique micro-focus in situ X-radiography method, which is impossible by synchrotron X-radiography or tomography methods. The nucleation undercooling and nucleation rate of PSPs have been measured. Besides, TP-1 type solidification test has been carried out. It is found that nucleation undercooling of PSPs is reduced, while nucleation rate and number density of PSPs is increased significantly by P inoculation. Moreover, the influence of cooling rate on the nucleation kinetics of PSPs in the P inoculated alloy was investigated in situ. It is observed that higher cooling rate has the influence of increasing the peak nucleation rate and extending the nucleation temperature ranges of PSPs, in terms of earlier nucleation at lower undercooling and nucleation stopping at higher undercooling, which results in higher number density of PSPs. The decrease in minimum nucleation undercooling with increasing cooling rate in the P inoculated alloy is unexpected, which has been attributed to the extremely slow growth rate of faceted Si crystal on AlP substrates under low undercooling.