The double-ring infiltrometer is widely used to measure soil saturated hydraulic conductivity. To investigate effects of ratio of the outer to the inner rings, i.e. the buffer index of double-ring infiltrometer, on measurement of saturated hydraulic conductivity, 16 sets of field experiments were laid out and conducted using 16 double-ring infiltrometers with 4 inner-ring diameters (20 cm, 40 cm, 80 cm, and 120 cm) and 4 buffer indices (0.2, 0.33, 0.5 and 0.71). Results show that uncertainty of the measured saturated hydraulic conductivity decreased with increasing diameter of the inner-rings. Saturated hydraulic conductivities in the infiltrometers with an inner-ring diameter of 20cm varied in a broad range, while those in the infiltrometers with inner-ring diameters of 120cm and 80cm were always close to each other and less in variation. However, no rising or falling tendency was observed for the increasing buffer indices. It is concluded that the inner-ring diameter is a more important factor to be considered than the buffer index in practice, and a larger inner-ring diameter, such as 80cm, is proposed as a lower limit of the inner-ring size to obtain reliable in-situ measurement of hydraulic conductivity.