A juice-extractor also serving as a grinder suitable for extracting juice from water-containing materials, such as vegetables, fruits, soybeans, and for grinding dry material into powder, such as coffee beans, green beans, nuts. In addition to elements similar to that of a juice-extractor, such as the funnel, the top cover, the lower grinding disc, the meshes-containing filter, the residue collector, the juice flow guider, and the base, the grinder also includes a bag made of air-pervious fabric and a rotational retainer having a closed side wall. When the juice-extractor is to serve as a grinder to grind a dry material into powder, the air-pervious bag is used to replace the residue collector and the rotaional retainer is used to replace the meshes-containing filter, so that powder produced from the grinding can be directly sent into the bag instead of the juice flow guider.