Abstract: Aiming at controlling herbicide pollution of the environment, biochars were prepared out of rice straw through oxygen-limited pyrolysis at low temperatures(200 and 350℃), and their effects of adsorbing and slow-releasing herbicide metolachlor were investigated. Results show that the biochar prepared at temperature of 350℃(D350) was 23.2 m²·g^-1 in specific surface area and hence much higher than rice straw in metolachlor sorption capacity, and close to the biochar prepared at temperature of 200℃(D200). But, biochar D350 showed a higher surface adsorption capacity to metolachlor and a higher desorption hysteresis index(5.35) than D200 did(2.07), indicating that granular preparation of biochar can be used as sustained releaser of metolachlor, with parameter n_r of its in-water release kinetic model being close to that(0.50) of the Fickian diffusion model, and the time it took to release 50% of adsorbed metolachlor(t₅₀) is positively related to the desorption hysteresis index.