Abstract. The moment of (p, q)-order, mp,q(C), of a circle C given by (x − a) 2 + (y − b) 2 ≤ r 2 , is defined to be C x p y q dxdy. It is naturally to assume that the discrete moments dmp,q(C), defined ascan be a good approximation for mp,q(C). This paper gives an answer what is the order of magnitude for the difference between a real moment mp,q(C) and its approximation dmp,q(C), calculated from the corresponding digital picture. Namely, we estimatein function of the size of the considered circle C and its center position if p and q are assumed to be integers. These differences are upper bounded11 +ε , where ε is an arbitrary small positive number. The established upper bound can be understood as very sharp. The result has a practical importance, especially in the area of image processing and pattern recognition, because it shows what the picture resolution should be used in order to obtain a required precision in the parameter estimation from the digital data taken from the corresponded binary picture. 1