Abstract: A strict equation of eccentric circle is offered in this thesis. Using this equation we get the expression of the coordinates of the least square circle center which is similar to the conventional formula, and the expression of the least square circle radius which is more precise than the conventional formula. Furthermore a formula of the value of roundness is offered in the sense of least square statistics and it is verified that it has nothing to do with the D. 0 component and the primary harmonic wave of the curve.

Key words: fitting circle, radius formula, roundness, eccentric circle