This review regarding prothrombin time and its standardization is described around some recent topics as the followings. 1. History of standardization for prothrombin time and revised WHO guideline for thromboplastin; A short history of standardization is summarized to understand a scheme of International Normalized Ratio (INR) based on International Sensitivity Index (ISI) that is calibrated by International Reference Preparation (IRP) for thromboplastin, and some key points in revised WHO guideline for thromboplastin and plasma used to control oral anticoagulant therapy are interpreted for research and practical use. 2. Point-of-care prothrombin time monitoring; A portable device to measure prothrombin time with whole blood sample, such as CoagChek (Roche), contributes to self-management by patients required long-term oral anticoagulation. Some investigators reported clinical agreement to use this monitoring system and improvement of patient's QOL and cost-effectiveness in overseas. 3. New types of thromboplastins; Two new types of thromboplastins have been available since the last year in Japan. One is a human plain thromboplastin, Simplastin HTF (Bioméreux) from extract of cultured human lung cancer cell, and another is IL test PT-Fibrinogen Recombinant (Iatron) from recombinant rabbit tissue factor relipidated in a synthetic phospholipid blend. For control of oral anticoagulation, good performance are expected in either thromboplastins because of their sufficient low ISI values. 4. INR methodology for other diseases; INR/ISI system is designed as a standardized methodology for control of oral anticoagulation. Prothrombin time, however is utilized as a global coagulation test for diagnosis or criteria of other disorders, such as congenital coagulation factor deficient, severe liver dysfunction and disseminated intravascular coagulation. Previous our study indicated that discrepancy of sensitivities to plasma absorbed multiple coagulation factors and plasma from patients under oral anticoagulation was revealed in rabbit brain thromboplastins, but not in human origins. Discrepancy of sensitivities observed in rabbit thromboplastins was emphasized in convert to INR values. These results suggested that the use of human thromboplastin of which ISI is close to 1.0 leads possibility for introducing INR methodology to evaluate PT of other disorders.