Objective To propose a new spectrophotometric method for determination of trace formaldehyde. Methods The method was based on the sensitive effect of formaldehyde in the system of cobaltous sulfate with ethylenediamine in sulfuric acid medium. Results The stable system of cobaltous sulfate with ethylenediamine could be kept seven days at least. Formaldehyde in the linear concentration range of 0.10-3.6 ?g/ml was determined with a detection limit of 0.16 ?g/ml, and the correlation coefficient was 0.997 6. Conclusion This method is simple, quick and has a high sensitivity and a good reproducibility. It has been applied to measure the given formaldehyde in tap water and free formaldehyde in UF resin with satisfactory results.