An ultrasonic time-domain reflectometry (UTDR) technique has been applied to the non-invasive study of fouling in micro filtration (MF) membrane modules operating at 100 kPa. The experimental results show a good correspondence between the UTDR signal response from the membranes and the development of a fouling layer on the membrane surface. The ultrasonic technique could effectively detect fouling-layer initiation and growth on the membrane in real-time. For experiments conducted at different axial velocities, ranging from 1.83 to 23.0 cm/s (Re from 65.4 to 811), a sequential mode fouling layer growth was observed. The UTDR technique was capable of distinguishing individual modes of growth. The data also suggest the formation of a second echo in the time domain at 100 kPa operating pressure. Therefore, the UTDR technique was used to quantify the thickness of a fouling layer. The non-invasive UTDR visualization of the fouling on the membrane surface was supported by "post-mortem" SEM analysis of the membrane surface. The UTDR technique can be more suitable for non-invasive visualization of the fouling in MF system.