Myelin Water Fraction uses the property that myelin water has shorter T2* relaxation time compared to axonal/extra-cellular water. Previous studies found that not only the T2/T2* relaxation time but also the T1 relaxation time is difference between these compartments. The T1 relaxation time of myelin water is known to be affected by the cross-relaxation therefore the term “apparent T1” is considered more accurate. In this study, we analyze the effect of this ‘apparent’ T1 in mGRE based MWF. Our results show that the MWF estimation is dependent on the differential T1 of different compartments. T1 effect on the MWF can makes overestimation error. Using the low flip angle could reduce the this error, but it gives rise to insufficient SNR on MWF. Also, increasing TR could be another choice for reducing estimated error, but it leads to inefficient scan time. Thus, GRE-MWF is needed to compensated T1 effect for accurate quantification.