The estimation of the initial oil in place is a crucial topic in the period of exploration, appraisal, and development of the reservoir. In the current work, two conventional methods were used to determine the Initial Oil in Place. These two methods are a volumetric method and a reservoir simulation method. Moreover, each method requires a type of data whereet al the volumetric method depends on geological, core, well log and petrophysical properties data while the reservoir simulation method also needs capillary pressure versus water saturation, fluid production and static pressure data for all active wells at the Mishrif reservoir. The petrophysical properties for the studied reservoir is calculated using neural network technique from 13 cored and logged wells. The results showed that the reservoir simulation method gave a value of Initial Oil in Place that agrees and close to the value of Initial Oil in Place obtained from the volumetric method with a percentage different around 2%. However, the estimation of Initial Oil in Place by reservoir simulation method offered accurate results during good history matching with observed data as well as making appropriate adjusting for Pc vs. Sw values for the whole reservoir from October 1976 until December2020. MB21 unit own most Initial Oil in Place equal to 525*106 SM3 while MB12 has lowest IOIP equal to 2*106 SM3. Finally, the calculation of Initial Oil in Place by both volumetric and simulation methods presented good results while comparing with previous study at 2013 with discovered different around 1.5% and 0.6% respectively.