Effects of boron and carbon on the densification and thermal decomposition of an AlN-SiC-$TiB_2$ system were investigated. $SiO_2$ was mostly removed by the addition of carbon, while $Al_2O_3$ formed $Al_4O_4C$ and promoted the densification of the systems above $1850^{circ}C$. Rather porous specimens were obtained without the additives after hot pressing at $2100^{circ}C$, while densification was mostly completed at $2000^{circ}C$ by using the additives. The sintering temperature decreased further to $1950^{circ}C$ by applying spark plasma sintering. The additives promoted the shrinkage of AlN by forming a liquid phase which was originated from the carbo- and boro-thermal reduction of $Al_2O_3$ and AlN.