Abstract: Based on Mie theory of light scattering of spherical particles, a novel homemade extinction cuvette was used to construct a handheld extinction photometer for in situ detection of milk adulteration without any complicated pretreatment of the milk samples. Adulteration of cow milk with various substances such as water, soup, powdered milk, soy milk, carbamide, heavy metal salts, etc. were in situ detected with minimal sample pretreatment. Surprisingly, the inferior milk adulterated with powdered milk, soup and/or other materials that could not be distinguished by a density meter was successfully differentiated in situ by the method reported. The detection limits of the method for spiked water and heavy metal such as mercury were lower than 2% and 0.005 μg/g respectively. The average time for single sample detection was less than 2 minutes using the current method, providing a practically useful tool for in situ gradation of milk quality.