Methyl methacrylate (MMA) was polymerized in a propylene carbonate–ethylene carbonate mixed solvent (50:50 wt%) containing LiClO4 (1 M) in the presence of a cross-linking agent, triethylene glycol dimethacrylate (TGD) or ethylene glycol dimethacrylate (EGD). The TGD–MMA (or EGD–MMA) molar ratio was 0–8%. A cell was assembled with the resulting gel and lithium electrodes, and was subjected to the electrolysis at 3 mA cm−2 for 1 h. The cross-linked gel electrolytes inhibited lithium dendrite formation. This inhibition effect is explained in terms of the mechanical properties of the gel. More rigid gels suppressed the dendrite formation more effectively, although a brittle gel may result in the formation of sparse, but relatively large dendrites. The EGD-containing gel whose EGD–MMA mol ratio was 4% successfully suppressed the dendrite formation.