Multivariate calibration based on partial least square regression (PLS) was applied for determination of calcium and iron in the flying ashes from the brown coal, using low-resolution XRF analysis. The measurements were performed using an AF-20 XRF analyser with the Ar proportional counter and the Cd-109 radioisotope source. With the range of CaCO₃ concentration from 20% to 60% the mean square error of crossvalidation was about 1.5% CaCO₃ and for Fe₂O₃ of concentration ranging from 3% to 12% the RMSECV was lower then 0.3% Fe₂O₃. It was found that application of the PLS calibration considerably reduces matrix effect. Applied calibration method as well as used measuring device can be used also for determination of strontium, which is sometimes present in the flying ash samples.