Scanning ion microscopy (SIM) employing focused ion beam (FIB) imaging was used to study the grain structure of thin copper films as a function of annealing temperature from 20 to 500°C. Accurate measurement of grain size is obtained for grains as small as 60 nm, allowing the microstructure of copper to be analyzed on small-grained samples which show poor contrast in scanning electron microscopy. Moreover, the short sample preparation time provides an advantage over transmission electron microscopy (TEM). The growth and coalescence of small (<100 nm) grains in the initially bimodal grain size distribution occurs in the temperature range of 250-350°C in films of 1000 nm thickness. This grain growth takes place concurrently with the relaxation of compressive stress as observed by temperature-ramped stress measurement. Also, temperature-ramped in situ TEM examination confirms that coarsening of small grains is the dominant grain growth mechanism up to 500°C.