The results obtained from efficiency tests conducted on a flat plate solar collector, according to the ISO 9806/1 test procedure, have been used to determine the uncertainty in the curve fitting parameters. The said standard, though requiring certain levels of accuracy in the measuring process, doesnot provide any method to determine the uncertainty of the efficiency curve parameters. The methodology used in the present paper (not provided by the ISO standard) allows solving the above mentioned problem and evaluating not only the parameters and their uncertainties but also the reliability of the test procedure and its goodness toward fitness. In order to evaluate the effects of measurement errors on the values of the uncertainty in estimated parameters, a sensitivity analysis has also been conducted. Strong dependence of some uncertainties, involving a larger accuracy level in the estimation of the measured parameters, is a clear indication of the present investigations.