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We have calculated formation of gas giant planets based on the standard core accretion model including effects of fragmentation and planetary envelope. The accretion process is found to proceed as follows. As a result of runaway growth of planetesimals with initial radii of ~ 10 km, planetary embryos with a mass of ~ [10.sup.27] g (~ Mars mass) are found to form in ~ [10.sup.5] years at Jupiter's position (5.2 AU), assuming a large enough value of the surface density of solid material (25 g/[cm.sup.2]) in the accretion disk at that distance. Strong gravitational perturbations between the runaway planetary embryos and the remaining planetesimals cause the random velocities of the planetesimals to become large enough for collisions between small planetesimals to lead to their catastrophic disruption. This produces a large number of fragments. At the same time, the planetary embryos have envelopes, that reduce energies of fragments by gas drag and capture them. The large radius of the envelope increases the collision rate between them, resulting in rapid growth of the planetary embryos. By the combined effects of fragmentation and planetary envelope, the largest planetary embryo with 21[M.sub.[symmetry]] forms at 5.2 AU in 3.8 x [10.sup.6] years. The planetary embryo is massive enough to start a rapid gas accretion and forms a gas giant planet. Keywords: Atmospheres, structure; Jovian planets; Origin, Solar System; Planetary formation; Planetesimals