  We investigate the effect of modified gravity on cluster abundance and the
Sunyaev-Zel'dovich angular power spectrum. Our modified gravity is based on a
phenomenological extension of the Dvali-Gabadadze-Porrati model which includes
two free parameters characterizing deviation from Lambda-CDM cosmology.
Assuming that Birkhoff's theorem gives a reasonable approximation, we study the
spherical collapse model of structure formation and show that while the growth
function changes to some extent, modified gravity gives rise to no significant
change in the linear density contrast at collapse time. The growth function is
enhanced in the so called normal branch, while in the "self-accelerating"
branch it is suppressed. The Sunyaev-Zel'dovich angular power spectrum is
computed in the normal branch, which turns out to be amplified compared to the
Lambda-CDM case. This allows us to put observational constraints on the
parameters of the modified gravity model using small scale CMB observation
data.
