Recently, numerous researches of overbased calcium sulfurized alkylphenates focused on their total base number (TBN) were reported. And it's well known that a increase of TBN(typically, greater than 300 mg KOH/g) was effective to reduce their cost. However, calcium sulfurized alkylphenates having a TBN greater than 300 have several problems on their unsatisfied performance, such as thermal stability.COSMO PETROTECH Co., COSMO RESEARCH INSTITUTE, and COSMO OIL Co. have developed a calcium sulfurized alkylphenate “PAD-450” having the TBN greater than 400mgKOH/g.“PAD-450” has similar performance with conventional calcium sulfurized alkylphenates (TBN =250mgKOH/g) on usual laboratory tests and a marine engine test. The innovational performance of PAD-450 is based on our specific procedure to increase TBN. In addition, we had examined a commercial process of PAD-450 with our plant that designed for calcium sulfurized alkylphenate (TBN=250mgKOH/g) and the results shows that the process is applicable for a conventional plant.We propose that PAD-450 is the first development of high-performance and high-TBN calcium sulfurized alkylphenates.