Rotary parts are used commonly in the field of precision machinery and their roundness error can affect installation accuracy greatly which determines the performance of machine. It is essential to establish a proper method for evaluating roundness error to ensure accurate assessment. However, various evaluation algorithms are time-consuming, complex and inaccuracy which can not meet the challenge of precision measurement. In this paper, an improved area hunting method which used minimum zone circle (MZC), minimum circumscribed circle (MCC) and maximum inscribed circle (MIC) as reference circle was proposed. According to specific area hunting rules of different reference circles, a new marked point which was closer to the real center of reference circle was located from grid cross points around previous marked point. Searched area was decreasing and the process of area hunting terminated when iteration accuracy was satisfied. This approach was realized in a precision form measurement instrument developed in NIM. The test results indicated that this improved method was efficient, accurate and can be easily implemented in precision roundness measurement.