Microhannel heat exchangers with louvered folded fins are subjected to frequent frost and defrost cycles when used in outdoor evaporator coils of air-source heat pump systems. Despite their potential for heat transfer enhancements, understanding the root causes that penalize microchannel heat exchangers during frosting operating conditions is still an open question. This thesis presents a new experimental methodology to study frost growth on louvered folded fins of microchannel heat exchangers. The paper demonstrates the feasibility of the frost mass and frost thickness measurements and, through a series of controlled laboratory experiments, provides new data of frost growth on louvered fins. Fin surface temperature was directly measured during actual frosting operating conditions and relations with heat transfer rates, air-side pressure drops, and frosting cycle times are discussed. The experimental results suggested that the frost mass increased linearly with time and water vapor did not experience any measurable time delay before it began to frost and to accumulate on the fins. The air-side flow blockage due to frost accumulation on the front leading edge of the fins was strongly dependent on the fin surface temperature and the data confirmed that the flow blockage was the key factor for the heat transfer capacity degradation during frosting operation.