Muon-spin-relaxation (mSR) technique is a sensitive magnetic probe to investigate electromagnetic properties of materials.The muon is a particle and has a spin half. The muon is injected from outside to materials in order to sense electronicproperties from the microscopic view point. One big advantage of the muon for material sciences is that the muon has the selfspin-polarization from the beginning of its life. Thus, we can carry out spin-resonance and relaxation measurements even inthe zero-field condition. RIKEN is one of the biggest national institutes of Japan and has established the RIKEN-RAL MuonFacility in the UK in 1994 . Word wide intense pulsed muon beams are available at this facility for material sciences and lots ofinternational/domestic collaborations are now going on by using muons applying to variety of materials. In my talk, I am goingto introduce the RIKEN-RAL Muon Facility, what is mSR and how we can make use of this research technique for materialsciences. And then, I am going to report recent our mSR results on nano-gold particles.Nano systems sometime show different electronic properties from those of bulk ones. For instance, the pure gold is expectedto show a magnetic ordered state in the nano-particle state. The mechanism to appear magnetic moment in the nano state of themetal gold is still unclear but some experimental evidences to show a possible ferromagnetic ordered state have been reported. It has been discussed that the magnetism in those nano-gold particles are fairly depends on the synthesis procedure ofnano particles and also depends on the size of the particle. We used a well size-controlled nano-gold particle to approach to thisproblem. Because the magnetic moment is expected to be very tiny, mSR is an ideal tool to sense such small magnetic moments.We have chosen the middle-size nano-gold particle which contains 144 gold atoms. The size of the particle is around 1.7 nm.The first report of the observation of a possible ferromagnetic ordered state has been done in this size . This size is recentlywell considered to be sitting in between the nano characteristics and the bulk one, so that our mSR study can give the criterionto understand how the magnetism appears in the nano-gold particles from what size of the particle.