Logistics and Physical Distribution are the important elements of customer Service mix.Seeing that we discuss service. we must clear what serviceability is. However.for the serviceability there is not yet a strict definition that authorized scientifically and generally. While.as to the maintainability concerning with the serviceability.there is maintainability engineering concerning with reliability engineering.And it is just logistics resource that supports those of serviceability. maintainability.and reliability.in a broad sense.<BR>From this points of view, serviceability, maintainability, and reliability are considered to be much affected by logistics resource availability. As a results, product and/or system availability depends on logistics availability too. In this sense, logisticsis is the important element of the dependability which is comprehensive concept including reliability, maintainability, serviceability, and availability.<BR>Because the dependability of product and/or system cannot be assured unless the supportability with logistics resources are sure to keep. As a reliability concept in a wide sense. IEC (International Electrotechnical Commission) has defined the dependability in IEC300 below.<BR>Dependability : The collective term used to describe the availability performance and its influencing factors : reliability performance. maintainability performance. and maintenance support performance.<BR>On the aspects mentioned above, the logistics supportability to the customer is considered as the most important element, Accordingly, the dependability in physical distribution can be recognized as the comprehensive term to be used describing logistics availability characteristics and the elements affects it, such as;<BR>(1) item demand rate characteristics.<BR>(2) inventory service support rate characteristics, and<BR>(3) reliability characteristics d maintainability characteristics of delivery service.<BR>Then, the author considered the concept of physical distribution logistics de pendability consists of physical distribution logistics service and its system effectiveness, and can be measured and assessed its performance, and tried to define below.<BR>Physical distribution logistics dependability<BR>-Physical distribution logistics service : <BR>All of the processes and activities that complete to deliver the available items which are required by customer on the given conditions within fixed time, and meet the customer's demand.<BR>-System effectiveness of physical distribution logistics : <BR>The probability that keeps available the items that are requested by a customer on the given conditions during fixed time, and meets customer's demand.<BR>From these point of view, the author studied supportability (demand fill rate) in relation to reliability and maintainability, focussing on the support function which is the important aspect of logistics in a broad sense. The author formulated supportability and support rate as measurement of supportability, and as a result, created a supportability theory model. Furthermore, introducing that consideration, the author added new understanding on traditional inventory service rate which was the determinants of logistics availability on the logistics dependability mentioned above, and formulated with the model that uniformed traditional inventory model and supportability model on probability theory. Expanding the concept, this paper presents the formulation of new service rate and logistics dependability and its optimizing technique.