Outer ring tilted or offset axially, caused by improperly fixing relatively to the inner ring, will produce remarkable systematic errors in measured radial clearance of a bearing. Analyzing their effects in detail on the results of measured radial clearances, they are found to be the main sources affecting the measurement accuracy. Measures for removing them are proposed. Based on these, a novel type of instrument for measurement of radial clearances of ball bearings has been developed. It could avoid the two kinds of systematic errors. The measuring principle, structures and working procedures of the instrument are presented. Test results show that the test time is less than 15 seconds per a part and the indication stability is between ±1.0 µm.