To provide a glass fiber sizing agent which can materialize high sizing properties to control the breakage of glass fibers when a matrix thermoplastic resin and the glass fibers are mixed, the glass fibers coated with the glass fiber sizing agent, and a glass-fiber reinforced thermoplastic resin material using the glass fibers.

The glass fiber sizing agent, which is applied on the surfaces of the glass fibers, includes a copolymerization compound and aminosilane and is neutralized with an amine compound having a boiling point of at least 170°C so that the pH of the glass fiber sizing agent is 5.0-8.0.