Phase separation of a ternary mixture composed of poly(ethyl acrylate) and poly(ethyl methacrylate) derivatives dissolved in methyl methacrylate monomer was induced and driven by photo-polymerization. Upon varying the initial composition of the mixture and the molecular weight of the resulting poly(methyl methacrylate) via changing the light intensity, a wide variety of morphologies ranging from separated double droplets, Janus-like droplets, core-shell, salami, bicontinuous to tricontinuous structures emerged due to the competition between phase separation and polymerization. Among the emerging morphologies, the tricontinuous structures were significant, and the details of the formation process were particularly investigated by using laser-scanning confocal microscopy (LSCM). Time-resolved experiments reveal that these tricontinuous structures were generated via two steps of the consecutive phase separation process. The formation of these tricontinuous structures is discussed with respect to the polymerization kinetics, the resulting molecular weight, and the residual monomer in the polymerizing mixture.