Background/Purpose. The biocompatibility and cytotoxicity of formaldehyde and monomer are essential in resin-based denture’s byproducts. This present study was performed to compare the release of formaldehyde and monomer and biocompatibility of three brands of heat-curing acrylic resins, including Ivoclar, Bayer, and Acropars, with different mixing properties and the same processing methods. Materials and Methods. In this experimental in vitro study, 18 samples were fabricated from Ivoclar, Bayer, and Acropars heat-curing acrylic resins (each group consisting of 6 samples). The released formaldehyde and monomer level were measured and registered for 1, 7, and 30 days. Also, methyl methacrylate release from samples was used to test cell cytotoxicity using L-929 murine fibroblast. The data were analyzed with repeated measures, one-way ANOVA, and Kruskal–Wallis tests. Results. For formaldehyde release of 1 day, Ivoclar acrylic resin showed the lowest level, followed by Bayer and Acropars acrylic resins ( P < 0.05 ). On 7 and 30 days, Bayer acrylic resin released the lowest formaldehyde, followed by Ivoclar and Acropars acrylic resins ( P < 0.05 ). Acropars showed the weakest and most significant results regarding biocompatibility and monomer release in all three points of time, respectively ( P < 0.05 ). Conclusion. Acropars acrylic resin showed the most significant formaldehyde and monomer release and least biocompatibility compared to Bayer and Ivoclar for 1, 7, and 30 days; however, after 30 days, all three resins displayed the same amount of formaldehyde release.