Purpose: To investigate effects of material thickness, curing light strength, and curing time on the cytotoxicity of a resin modified glass ionomer (RMGI). Methods: A total of 112 specimens of a RMGI (Vitrebond, 3M ESPE, Saint Paul, MN) of 1-, 2- and 3-mm thickness were prepared using 1:1 mixing ratio of powder and liquid, following manufacturer's instructions. The specimens were light-cured on the top-side for 15-, 30- or 45-second using three curing lights of the measured power of 400, 800 and 1,000 mw/cm2, respectively. The cytotoxicity of the samples was evaluated using the agar diffusion method (ISO 7405). L929 Cell suspensions of 1.0x105 /mL were aliquotted into 6-well plates and cultured for 24 hours at 37°C in a humidified atmosphere of 5% CO2. The media was withdrawn and an overlay agar was poured over the cell monolayer. Cells were stained with neutral red solution. One-hour after the curing, the specimens (n=4) were placed at the center of the agar surface. Culture medium and phenol served as negative and positive controls, respectively. The cytotoxicity was evaluated after 24- and 48-hour. Results: No cytotoxicity was detected in any of the 2-mm specimens cured with ≥800 mw/cm2 light. Specimens of 1-, 2- and 3-mm thickness cured with 400 mw/cm2 light for 15-second induced mild, moderate and severe cytotoxicity at 48-hour, respectively. Regardless of curing time, all 3-mm specimens cured with 400 mw/cm2 light caused severe cytotoxicity, except moderate cytotoxicity detected at 24-hour on 45-second samples. When light-cured for 15-second with 800 and 1,000 mw/cm2, the 3-mm specimens resulted in moderate and severe cytotoxicity at 48-hour, respectively. Conclusions: The RMGI of 2-mm thick or less is not cytotoxic when cured using a light of ≥800 mw/cm2. Material thickness, curing time and strength of the curing light may influence the cytotoxicity of RMGI.