This document describes a method in laboratory to assess the performance of ultraviolet germicidal irradiation (UVGI) devices which will be mounted in-duct in heating, ventilating and air-conditioning (HVAC) systems. The method includes the detailed requirements for test rig, microorganisms, procedures, data calculation and result report to determine the UV dose to model microorganisms by an UVGI device at several airflow rates. By the testing results, the capacity of in-duct UVGI devices for air disinfection can be evaluated and compared reliably. If the susceptibility constant of a given microorganism is known, the inactivation rate of that micro-organism by the tested UVGI devices can be further calculated.