The effects of liquid-type nucleating silane additives on the cell structure, mechanical strength, and thermal insulating properties of the polyisocyanurate (PIR) foams have been studied. The PIR foams synthesized with hexamethyldisilazane (HMDS) as a silane additive showed the smaller average cell size and lower thermal conductivity than those of the PIR foams prepared with the hexamethyldisiloxane, dimethoxydimethylsilane, and hexadecyltrimethoxysilane. When HMDS was added, average cell size of the PIR foam was becoming smaller due to lower surface tension of the polyol solution, thereby the nucleation rate and number of bubbles produced were increased and then the cell size becomes smaller. The additives likely act as nucleating agents during the formation of PIR foams. The smaller cell size appears to be one of the major reasons for the improvement of thermal insulation properties and mechanical properties of the PIR foams. From the results of cell size, thermal conductivity, and mechanical strength of the PIR foams, it is suggested that the HMDS may be the efficient liquid-type additive for the reduction of cell size and improvement of the thermal insulation property of the PIR foams.