Thallium (Tl) is a priority pollutant with high toxicity and exposure risk. With rapid development of high-tech industries, technology-critical elements such as Tl could be released into the paddy soils via use of manufacturing wastewater as irrigating water sources for rice culture. The Tl occurrence in contaminated paddy soils was detected in previous studies; however, the pollutant transportation, bioavailability and environmental toxicity of Tl in contaminated paddy field are less known. In this study, we aim to use medaka fish as a model to assess the bioavailability and toxicity in Tl-contaminated paddy soil. Medaka embryos were exposed to different concentrations of Tl (0-1000 mg/L). The rates of survival, hatching and heart-beating were measured in embryos within 6 days of exposures. The locomotor activities of hatchings from treated embryos including average velocity, percentage time active, relative turn angle, absolute turn angle and maximum velocity were analyzed at 3 day-post-hatching. The results showed that the 6-day LC₅₀ value of Tl was above 1000 mg/L. The heart-beating rates were significantly reduced for concentration ranging from 200 to 1000 mg/L as compared to the control. The hatching rates were not significant as compared to the control. The subsequent research will establish a biological exposure system for stimulating Tl-contaminated paddy fields to evaluate the Tl bioavailability from soil to medaka fish.