Green manures, often known as cover crops, are plants which are grown to improve the soil structure and nutrient content and organic matter of the soil. Sunn hemp (Crotalaria juncea) could be a valuable green manure or cover crop as source of soil organic matter for farmers as crops producers because of its rapid growth and large nitrogen fixing ability. The time at which a green manure crop should be buried is the most important in deriving the full benefit from the green manure. The aim of this observation were to determine how old the sunn hemp has the highest contain of nitrogen, phosphorus, potassium and organic matter. After all, it has high quality to be used as green manure and soil organic matter. This research was conducted in two stages, field and laboratory observation. Field observation was conducted at Siman district, Ponorogo city, divided into cultivation, planting and pruning. While the laboratory experiments includes observation of nutrient contents of Nitrogen, Phosphorus, Potassium, organic matter and C/N ratio of sunn hemp, was done at Soil Laboratory, Brawijaya University, Malang, to determined nutrient contain of sunn hemp, was done at 15, 30, 45, 60 and 75 DAP (days after planting). The results of this experiment showed that sunn hemp has variant nutrient content during each observations. Furthermore, C. juncea at the age of 15, 30, 45, 60 and 75 DAP all can be used by producers and farmers as green manure and valuable source of soil organic matter, based on SNI No. 19-7030-2004