The validated protein precipitation method was applied for estimation of Abemaciclib in human plasma with Abemaciclib-D10 as an internal standard (ISTD) by using HPLC-ESIMS/MS. The chromatographic separation was achieved with Methanol: Acetonitrile (pH: 6.5 Adjusted with diluted ammonia solution) (20:80%, v/v) using the Discovery® C18 HPLC Column, 2 cm × 2.1 mm, 5 μm. The total analysis time was 3.0 min and flow rate was set to 0.7 ml/min. The mass transitions of Abemaciclib and Abemaciclib-D10 obtained were m/z 507.32 393.16 and 517.66393.16. The standard curve shows correlation coefficient (r2) greater than 0.9983 with a range of 6.00-768.00 pg/ml using the linear regression model.