Immobilization of highly monodispersed palladium nanoparticles in colloidal mesoporous silica supports has been successfully achieved. The Pd nanoparticles with a uniform small size of 1.2 nm can be homogeneously distributed in individual mesopore channels of amino group-functionalized three-dimensional dendritic mesoporous silica nanospheres (3D-dendritic MSNSs) with a Pd content of 2.8 . The 3D-dendritic MSNSs-based nanoreactors show high activity in Suzuki-Miyaura cross-coupling reactions of bromobenzene with phenylboronic acid, obtaining a yield over 99 with 0.075 mol Pd catalyst at room temperature in the dark within 12 h. More importantly, the excellent catalytic performance can be maintained with a negligible decrease lasting at least six cycles. It further reveals that the mesoporous frameworks of the colloidal silica supports can be well-preserved after four catalytic runs; meanwhile, the Pd nanoparticles in the mesopore channels also can remain the sizes of 1.5 +/- 0.3 nm without significant transfer and aggregation. The unique mesostructure of the 3D-dendritic MSNSs with mesopore channels of short length and large diameter is supposed to be the key role in immobilization of active and robust heterogeneous catalysts, and it would have more hopeful prospects in catalytic applications.