The response of a sap flow meter for a one year old Eucalyptus tree, as a function of two characteristic design parameters of the transducer was evaluated. The variables tested were the electric power applied to the heater and the position of the thermocouples of the flow meter. One year old potted trees of Eucalyptus nitens with a 16 mm diameter trunk were used. In the flow meter, distances of 10 to 30 mm from the edge of the heater and heating power of 0.16 w to 0.48 w were tested. The sap flow was evaluated and compared with the results obtained in parallel by means of differential weight measurements of the potted trees. The best results were observed with heating power of 0.40 w and a thermocouple distance of 20 mm from the edge of the heater. The best results were obtained with a power of 0.40 w and a distance of 20 mm from the heater edge for the thermocouples. Even tough the results are only valid for small trees of dimensional features similar to the tested units; however the methodology to improve the design of a sap flow meter can be generalized for any particular kind of small tree.