The sol-gel transitions of linear low density polyethylenes in decalin, tetralin, o-xylene, and toluene were studied. Each sample was a copolymer of ethylene and butene-1, 4-methyl-pentene-1, or octene-1. A thermo-reversible gel was formed for each system and gel-melting temperatures were measured at concentrations of about 4–14 g/100 cm3. It was found experimentally that the gel-melting temperature depended on polymer concentration, molecular weight of the polymer, and the kind of comonomer. Moreover, the structure of crystallite involved in the gel was examined by a polarizing microscope and many spherulites were observed. The data were treated phenomenologically using the thermodynamic theory of Takahashi, Nakamura, and Kagawa, derived for the gel-melting temperature of a crystalline linear copolymer gel. In each solvent, a linear relationship was found to hold between the reciprocal absolute gel-melting temperature 1/Tmg, and ln V2N for samples having a same kind of comonomer, where V2 is the volume fraction of the polymer and N is the weight-average degree of polymerization.