An epoxy resin composition for semiconductor encapsulation in producing surface mount lead-less thin semiconductor devices. The epoxy resin composition for surface mount lead-less semiconductor device encapsulation which device comprising an encapsulating resin layer and, encapsulated therein, a substrate, a semiconductor element mounted on the substrate, two or more conductive parts disposed around the semiconductor element, and wires which electrically connect electrodes of the semiconductor element to the conductive parts, wherein the bottom face of the substrate and the bottom face of each conductive part are exposed without being encapsulated in the encapsulating resin layer, and the epoxy resin composition used for forming the encapsulating resin layer has the following properties (.alpha.) and (.beta.): (.alpha.) a melt viscosity of 2-10 Pa.s at 175.degree. C.; and (.beta.) a flexural strength of cured state of 130 MPa or higher at ordinary temperature.