The purpose of this study is to estimate the pressure disturbance area, or the propagation of the radius of investigation, from a producing well in an infinite reservoir by using both analytical and numerical methods. A linear ( or radius) coefficient in the relationship between the square of the dimensionless radius of investigation and the dimensionless time is studied and derived. The radius coefficient in the equation is a constant, and varies with different criteria of radius of investigation defined, i. e., the amount of pressure change from the initial formation pressure at the pressure front of the pressure disturbance area. As the dimensionless pressure defined at the pressure front changed from 0.1095 to 10(-9), the radius coefficient varied from 4 to 71.15. The radius coefficient is independent of the level of the flow rate for a well producing at a constant flow rate. For a well producing with variable flow rates, the radius coefficient is not a constant for the case of larger pressure drops defined at the pressure front. The skin factor does not affect the result of the calculated radius of investigation. The wellbore storage volume will affect the propagation of the radius of investigation only at an early time, depending on the wellbore storage volume.