The present invention provides, inter alia, a method for producing a masterbatch in which the dispersibility of microfibrillated plant fibers is further increased to obtain excellent processability and improved rubber properties such as fuel economy, tensile strength, and elongation at break. The present invention relates to a method for producing a masterbatch, the method including step (I) of mixing a rubber latex with microfibrillated plant fibers oxidized with an N-oxyl compound to obtain a mixture, and coagulating the mixture by adjusting its pH to 2 to 6.