A validated Ribociclib bioanalytical method was developed by HPLC-ESI-MS/MS using stationary phase Zorbax SB-C18, 4.6 × 75 mm, 3.5 μm, 80 Å column and mobile phase was used 5mM ammonium formate: acetonitirle (10:90 v/v). The Ribociclib and Ribociclib-D6 were identified using ESI mode at m/z 435.5 /112.1 and Ribociclib-D6 (IS) at 441.7/112.1. The correlation coefficient is ≥ 0.998 with linearity range 50.00-10000.00 pg/mL.