We have developed a convenient and effective XRF analysis procedure for trace amount of K, Ca, V, Mn, Fe, Ni, Cu, Zn, Cd, and Pb in environmental waters by using a preconcentration using the natural polymer (agar). The thin agar film was prepared by drying a homogeneous agar gel after mixing the aqueous sample solution with the agar powder. XRF analysis of the preconcentrated thin agar films containing trace metals showed a good repeatability, because agar films were homogeneous enough. SB (signal to noise) ratios of the XRF intensity of the analytes were improved drastically. The linear calibration curves of K, Ca, V, Mn, Fe, Ni, Cu, Zn, Cd, and Pb showed a good linearity within the calibration ranges. The lower limits of detection (LLD) were 1.4 µg/mL for K, 0.26 µg/mL for Ca, 0.088 µg/mL for V, 0.029 µg/mL for Mn, 0.11 µg/mL for Fe, 0.016 µg/mL for Ni, 0.030 µg/mL for Cu, 0.017 µg/mL for Zn, 0.20 µg/mL for Cd, and 0.066 µg/mL for Pb, respectively. The proposed preconcentration method was applied to several environmental water samples. 96