The performance of mechanical smoke exhaust systems in a small retail shop fire will be evaluated. A series of full-scale burning tests will be carried out and the results will be justified by numerical experiments. A zone model has been developed to predict the fire growth in small retail shops with different equations on axisymmetric fire plume entrainment and spill plume. In addition to verifying the developed model, the results will also be compared with those predicted by another zone model CFAST (version 5.01) and a field or computational fluid dynamics model Fire Dynamics Simulator FDS code (version 3). The correlation between the mechanical exhaust rate and plume entrainment rate for keeping the smoke layer interface at a certain height under a steady burning fire will be analyzed. Further, the application of different plume correlations will be investigated. The effectiveness of mechanical exhaust in retail shop fires will be discussed and some factors affecting the system performance will be analyzed.