In this paper, we describe the utilization of coco peat, commonly used in horticultural production, as a bulking agent for dairy cow manure composting. Coco peat has high water absorption and low moisture content, which are important factors for bulking agents; however, its utility in composting processes has not been clarified. Our objective of this study was to clarify the characteristics of coco peat as a bulking agent in composting material. Stainless fermenters with a volume of 430 L were used as an experimental device. The mixture of cow manure and coco peat was adjusted to 70%, 74%, 78%, and 82% moisture content, and cow manure and rice husks adjusted to 70% moisture content were used as control. As a result, the organic matter decomposition rate and compost temperature were low under the condition of mixing coco peat at the same moisture content as rice husk, and coco peat was not a significantly more effective bulking agent than rice husks. However, composting was possible even at high moisture contents above 75%, where fermentation stagnation can occur in rice husks. The higher moisture content using coco peat led to a higher temperature of the composting material and more organic matter decomposition because of more easily degraded organic matter content originating in manure. Moreover, ammonia emission and the decrease in fertilizer components were less in coco peat than in the control. Therefore, it can be concluded that coco peat can be used as an alternative bulking agent for dairy cow manure composting.