Abstract: An oil well in Darquain region of Khuzestan province in Iran was selected as a case study and contemporary changes in pressure and temperature according to real inside borehole conditions of this well have been simulated in curing the class G cement samples in the laboratory. The mentioned cement usually used for cementing the annulus between casing and rock formation in the oil well early age tensile strength of the samples were measured via Brazilian tension test in order to perform more precisely predicting tensile cracking of cement sheath. It is worth to mention that in this study no special additives used in cement slurry mixture in order to evaluate only the effects of environmental conditions in curing the cement samples. Results of laboratory tests showed that with the rise of the pressure and temperature up to 150°F and 1500 psi, earl age tensile strength is growing slightly while after that it experiences a decreasing treatment.