Abstract: A method for the field in-situ determination of trace iron in natural water and chemical exploration samples by micro-bead colorimetry was developed. Based on the chromagenic reaction of iron with 4,7-phenyl-1,10-phenanthroline (apparent molar absorptivity is 1×105 L·mol-1·cm-1） and Na2S2O3 as a masking reagent for eliminating the interference from Cu, the method provides detection limit of 0.001 μg/g for Fe. The method has been applied to the determination of trace iron in natural water and chemical exploration samples and the results are in agreement with those from spectrophotometry with precision of 2.5％RSD（n＝10）. But in the analysis of samples with higher content of Fe, the sample solution should be further diluted. The method also provides the advantages of higher sensitivity, accuracy and precision, low cost and less environmental pollution. 