The present invention relates to a method for composting organic material, in which method organic material is transported to a bioreactor (1) where it is mixed and where material for regulating the oxygen content and bacteria and/or enzymes for agitation of composting process are added to it, and where organic material may compost, after which the composted material is removed outside the bioreactor to be stabilized. The invention also relates to a device for composting organic material, which device includes a bioreactor (1) with at least one filling and emptying opening (12, 13, 17) in order to fill and empty the bioreactor, and a transporting device (2) mixing the material inside the bioreactor (1) in order to transport the organic material to be composted and a mix it inside the bioreactor. Characteristic to the method in accordance with the invention is that organic material is transported by means of a mixing transporting device (2) upwards and then dropped down against a spreading element (16) and from there back to the transporting device thus spreading the organic material to a smaller particle size and the particles while falling down come into contact with oxygen and moisture of the air in the bioreactor (1). Characteristics to the device in accordance with the invention is that the bioreactor (1) includes a turning element (11) to direct the organic material to be transported with a transporting device (2) upwards and back to mainly opposite direction in respect with the transporting direction of the transporting device.