The single crystals of Yb3+-doped and Si4+ co-doped Y3Al5O12 (YAG) were grown by the Czochralski method. Both crystals belong to the cubic system with the Oh10-Ia3d space group. The addition of Si4+ does not change their crystal structure, but changes the luminescent ions valent. The absorption spectrum manifests that the substitution of Al3+ by Si4+ leads to the transformation of Yb3+ to Yb2+ for the charge balance, and then reduces the luminous intensity of Yb∶YAG. The X-ray and γ-ray excited luminescence (XEL) measurement shows that the luminous intensity of Si4+ co-doped Yb∶YAG is about 63% and 40% of Yb∶YAG, respectively. Additionally, because of the isotope of Yb, Yb∶YAG can even be excited by neutrons with nuclear reaction, the charge particles cause secondary reactions to produce fluorescence. The fluorescence intensity is still determined by Yb3+, thus the sensitivity for neutron is also reduced by Si4+ co-doping.