This paper employs physical modelling technique to test an outfall structure, as a new approach to streams with low flow velocity and poor water mixing. From available data at the Hydraulics Research Institute (HRI), a thermal power plant close to the left bank of El-rayah El-tawfiki was taken as a case study (Banha power plant) to test a series of outfalls with different combinations. A physical model was constructed inside the northern hall of HRI. One surface outfall and a series of multi-port diffuser outfalls with different parameters were tested. The tested outfall parameters comprised number of diffuser rows, nozzle diameter, space between nozzles and their number. The tests were carried out with one or two rows of the diffuser in operation. The results indicate that the surface outfall leads to a thermal plume with temperature rise more than 5˚C above ambient water outside the mixing zone. For the multi-port diffuser outfall there is high capability of diluting the temperature rise to less than 5 o C outside the mixing zone and confining the plume in the third of the width of the stream. The diffuser which has two rows in operation has achieved almost the same mixing zone area as in case of diffuser with only one row in operation near the bed level. The results indicate that it is better to use a diffuser with one row only near the bed level to avoid high cross flow and surface turbulence which may obstruct fish boats operating nearby.