The two-dimensional Pt submonolayers on Au(111) were used as model catalyst system to study kinetics of hydrogen oxidation reaction (HOR). The morphology of different Pt submonolayers was characterized by ex situ scanning tunneling microscopy combined with statistical image analysis. The HOR kinetics data were analyzed using Levich-Koutecky formalism and presented as a function of the mean size of Pt clusters for each Pt submonolayer. The Pt submonolayers with smaller Pt clusters were found less active for HOR. This trend is well correlated with the continuum elasticity analysis of the average active strain in Pt clusters indicating that smaller clusters have less tensile strain. The density functional theory calculations were found in agreement with our results demonstrating that the size-dependent strain in Pt clusters has significant effect on the energy of the d-band center, i.e., the Pt clusters' activity.