Abstarct: Creatinine is the degradation product of creatine, the very popular muscle-building supplement. Creatinine is produced (from creatine) inside the cell, in the blood stream, in your glass of creatine before drinking, or on the shelf in your creatine container. Basically, creatine will spontaneously convert to creatinine in almost any environment you can image. Investigation of tautomerism in creatinine is very important in biology. The absence of experimental energetic and structural data of tautomeric equilibrium presents a challenge to quantum chemical methods to obtain an insight into the reactivity of these compounds. The geometries of various tautomers and isomers of creatinine have been studied using the B3LYP/6-311++G** method. Relative Gibbs free energies can be used to obtain ∆G and Keq between tautomers and ∆G# and rate constants for tautomerism interconversions of each molecule.،The structure of transition state between each pair of tautomers was optimized by appling Schlegels synchronous transit-guided quasi-newton (QST3) method started from the fully optimized structure of one tautomer and finished on the fully optimized structure of another tautomer.The transition states were verified with frequency calculations to ensure they were first order saddle points with only one negative eigenvalue. Additionally intrinsic reaction coordinate (IRC) calculations proved that each reaction lixnked the correct products with reactants.Rate constant were calculated by canonical transition state theory using eyring equation. In the gas phase 2-imino-1-methyl imidazolidin -4-one (Keto-form) are computed to be more stable than the other tautomers. Then, To consider solvent effect, self-consistent reaction field method (SCRF) and polarized continuum model (PCM) have been used. In this research ,five tautomers of Creatinine and E/Z Isomers and the equilibrium reactions between in gas phase and various solvents, such as Water, Methanol,Acetone,THF, Chloroform and Cyclohexane that they have different dipole moment and different dielectric constant, have been studied From investigation for different tautomers in various solvents , is conclude that , dipole moment and dielectric constant of each solvents has influence on dipole moment and stability of tautomers. In The Solvents, 2-amino-1-methyl-4,5-dihydro-1H-imidazol-4-one are computed to be more stable than the other tautomers.