The outstanding permanent magnetic material Nd2Fe14B possesses very high energy product. But its Curie temperature is a little bit too low for practical application. Some elements were added into the material in order to raise the Curie temperature. Reference [9] and our recent experimental results show that the Curie temperature can be made higher by partiy subsituting silicon for iron in the material and in Y2Fe14B. For understanding the effect of silicon on magnetism, the occupancy of Si in Y2Fe14B was studied by neutron diffraction. The neutron diffraction results show that silicon occupies preferentially the sites of c, j1, and k2. The magnetic interaction theory is applied to explain the experimental results.