We have investigated the intrinsic gettering (IG) of Cu in silicon wafers. Oxygen precipitates for IG were formed in wafers subjected to two-step annealing consisting of thermal treatment for 16 h at 800degreesC, then for 0-16 h at 1000degreesC. Cu was deposited on the back surface by dipping the wafers into a contaminant solution, and introduced into the wafer bulk from the back surface by heating for 5 min at 1000degreesC. The surface concentration of Cu was measured by total-reflection X-ray fluorescence. We found that the gettering efficiency, which was determined from the difference between the surface concentration on reference wafers and the corresponding concentration on the IG-treated wafers, depends on the Cu contamination concentration. Moreover, we used a simulation based on the Fokker-Planck equation to analyze the data with regard to the dependence of gettering efficiency on oxygen precipitate density. As a result, we have shown that the total number of oxygen precipitates, rather than their size determines the gettering efficiency. These experimental and calculated results can provide useful information for achieving effective IG of Cu.