To provide a formalin-free glass fiber sizing agent for producing glass paper of high solvent resistance and heat resistance, and to provide the resulting glass paper.

The glass fiber sizing agent comprises (A) an aqueous resin obtained by polymerizing a monomer component including no (a-1) carboxy-containing monomer component and essentially comprising (a-2) a tertiary amino group-containing ethylenically unsaturated monomer and (a-3) a hydroxy-containing ethylenically unsaturated monomer in the presence of (A') a carboxy-containing polymer with the monomer component(a-1) as the essential component, (B) a compound containing silanol group and/or hydrolyzable silyl group, and (C) a polyisocyanate resin and/or a blocked polyisocyanate resin. The glass paper obtained using the glass fiber sizing agent is also provided.