A linear frequency-thickness (F-T) relation was established for surface-initiated atom transfer radical polymerization (SI-ATRP) in a quartz crystal microbalance (QCM). This quantitative F-T relation is monomer dependent but independent of polymerization rate, initiator and polymer density. With this F-T relation and the online monitoring capacity, QCM was successfully applied to study the kinetics of SI-ATRP mechanisms. QCM was also demonstrated to be useful in controlling film thickness at the angstrom level, which is critical in nanofabrication.