Abstract: C/ C-SiC composites were fabricated by molten silicon infilt ration (MSI) to C/ C preforms prepared bychemical vapor infilt ration (CVI) . The influences of adding Al in molten Si infilt rating agent on the microst ructureand mechanical properties of the composites was studied. The result s show that the composites fabricated by infiltrating Si into C/ C preforms contain SiC , C and residual Si phases. With the increasing of the amount of adding Al inthe agent , the amount of Al phase in the composites increases and the amount s of other phases such as SiC and Siphase decrease. SiC phase dist ributes around the carbon f iber , and Si phase exist s far away from the carbon f iberwhere there is nearly no Al phase. The bending st rength and the f racture toughness increase with the increase of theamount of adding Al. When the amount of Al increases from 0 to 10 % , the bending st rength increases from 116. 7MPa to 175. 4 MPa and the f racture toughness from 5. 8 MPa·m^1/2 to 8. 6 MPa·m^1/2 , that is , by 50. 3 % and 48. 2 %respectively. The Al phase makes the f racture mechanism of the composite become toughness f racture.