There is provided a varnish of a low dielectric loss tangent resin composition which is low in viscosity and excellent in storage stability. The varnish of a low dielectric loss tangent resin composition contains a thermosetting monomer (A) having a weight average molecular weight of not more than 1,000, a high polymer (B) having a weight average molecular weight of not less than 5,000, a halogen flame-retardant agent (C), a silicon dioxide filler (D), and an organic solvent (E). The average particle diameter of each the components (C) and (D) is in the range of 0.2 to 3 .mu.m.