The objective of this study is to provide the best practice guide for loading of in-service aged power transformers including not only temperature, but also loadinghistory, moisture, and possibly maintenance practice. The secondary objective of thisstudy is to research, develop and determine life expectancy and loss of life curves foraged transformers considering above mentioned factors. This research work focuses onliterature reviews including any standards (IEEE, IEC, ANSI, etc.) to explore variousapproaches to develop loading guide for aged power transformers. The study includesa simplified spreadsheet simulation to develop loading guide and provide the estimatelife expectancy and loss of life of a transformer considering various parameters. Utilities involved will be surveyed to incorporate the industry practice.