Accurate estimation of the postmortem interval (PMI) is an important research topic in Forensic Medicine,but all methods till now in use to determine it are unreliable. Vitreous humor of eye being relatively stable, less susceptible than other body fluids to rapid chemical changes and contamination, easily accessible and is suitable to estimate postmortem interval. Aim of the study: to assess the accuracy of vitreous potassium (K) level measurement in estimating PMI. Material and Methods: This study was carried out on cases brought to the mortuary of Benha medicolegal department of Ministry of Justice, Qaluobyia, Egypt, during "March 2012 to October 2012". Vitreous humor samples were collected from both eyes of deceased  for determination of K levels. Actual postmortem interval was estimated according to the history and changes recorded in the cadavers. Results: the total number of cadavers was 120; 77 males and 43 females (ratio 1.8:1). There was significant increase in the potassium concentration with increasing PMI. The average rate of increase of vitreous potassium was calculated as 0.10795 mmol/L per hour. Moreover, there is linear correlation between vitreous K and PMI. There was no significant effect for variation of gender, age and temperature on vitreous K level. There is a significant correlation between K levels and actual PMI from police records, hospital records and postmortem changes. There is no significant statistical difference between actual and estimated PMI. Conclusion: there is a linear pattern of increase in vitreous potassium level with PMI and it can be used as a valuable indicator to estimate PMI accurately in cases of deficient data about this time from other methods.