The book has been divided into nine chapters. It deals the introduction to differential equation, differential equation of first order but not of first degree, the differential equation of first order and first degree, application of first order differential, linear equations, methods of variation of parameters and undetermined coefficients, linear equations of second order, ordinary simultaneous differential equation, total differential equations (Pfaffian Differential Forms and Equations). The book include fundamental concepts, illustrative examples and applications to various problems.