The presence of flat bands is a source of localization in lattice systems. While flat bands are often unstable with respect to interactions between the particles, they can persist in certain cases. We consider a diamond ladder with transverse hopping that possesses such stable flat bands and show that many-body localization appears in the presence of interactions. We demonstrate that the eigenstate thermalization hypothesis is violated and verify localization by time evolution of local observables, revival probabilities, and participation ratios. Thus, this system appears to be an example for many-body localization without disorder.