A toner for developing electrostatic images includes: toner particles and organically treated alumina powder. The organically treated alumina powder has an X-ray diffraction characteristic showing a maximum X-ray intensity level I.sub.a-max and a minimum X-ray intensity level I.sub.a-min in a 2.theta. range of 20 to 70 degrees providing a ratio I.sub.a-max /I.sub.a-min of below 6. The alumina powder is amorphous or has a low-crystallinity of .gamma.-form, thereby showing a low agglomeratability to function as an effective flowability-improving agent for a toner. The structural water contained in the alumina powder contained favors hydrophobization treatment thereof and functions to suppress a charge-up phenomenon in a low humidity environment after the hydrophobization.