PURPOSE: To determine a coagulation factor easily and accurately, avoiding the influence of existing colored materials, by treating a substrate with an enzyme, diazotizing and color-developing the isolated p-nitroaniline or its derivative, and determining the product by colorimetry.

CONSTITUTION: In the determination of coagulation factor by the use of a substrate composed of modified p-nitroaniline or its derivative, the p-nitroaniline or its derivative isolated from the substrate is diazotized with a nitrous acid salt and a coupling component. Preferably the excess nitrite is inactivated with an inactivation agent such as sulfamic acid, urea, etc. under acidic condition, and the color developed by diazotization is determined by colorimetry.