The environmental conditions in which the equipment is to be operated, such as temperature, humidity, dust, operators’ skill, etc, often have considerable influence directly on the product reliability and indirectly on the product supportability characteristics. This paper, after discussing the factors influencing product reliability, describes a method to estimate spare part requirements based on estimation of the actual reliability of a product under the influence of the product-operating environment using a proportional hazard model. In this research only non-repairable components/parts in repairable systems are studied. Results express a considerable difference between considering and ignoring the operating environmental factor on system performance. So, this factor should be seriously considered while dimensioning product support and service delivery performance strategies, since it will have an impact on operation, maintenance and service quality.