In condensed matter physics, a little frustration can make materials interesting. This is because that the presence of competing forces, which cannot be simultaneously satisfied, can give rise large degeneracy to the ground state of the corresponding system. One group of such interesting matters are the geometrically frustrated system. Ba3CoNb2O9 powder sample exhibits a triangular lattice and frustrated magnetic structure. In contrast, our recent study found that the single crystal of Ba3CoNb2O9 has a cubic structure and that the spins does not show a well-defined long-range order. These observations imply that Ba3CoNb2O9 single crystal might be a candidate of spin liquids. However, further evidence are required. In this proposal, we would like to use the µSR technique to measurement the magnetic properties of the Ba3CoNb2O9 single crystal.