In this work, four investigations of stratified flow were conducted in closed conduits, which included, l. numerical solution of velocity distribution by using a digital computer, 2. analytical solution and experimental verification of the internal surge wave velocity, 3. experimental observation on the mutual intrusion phenomena, and 4. experimental investigation of the internal hydraulic jump. An apparatus consisting mainly of a total length of 13 feet of transparent plexiglas pipe, two liquid pumps, a separator, two constant head tanks, a receiving tank, flow control valves, manometers, and discharge measuring devices, was built for the above investigations. The equation of motion was derived for the velocity distribution, starting from the Navier-Stokes equations. A special Poisson equation with a constant term on the right hand side of the equation was derived which was then reduced to a Laplace equation by a substitution. Considering appropriate boundary conditions, this equation was solved by a method of finite differences on an IBM 7040 computer. The method used in computation is solving a set of simultaneous equations by elimination. Less than two minutes machine time was required for a 39 by 39 matrix. Experimental results demonstrated fairly well the energy loss concept in surge wave considerations. The major energy loss at the wave front was assumed to be the expansion loss of the lower layer. The approximate solution holds for internal surge waves with small amplitude of any flowing area. A non-antisymmetric property was found during the mutual intrusion. That was quite contrary to the existing literature. No transient effect was apparent due to the limited length of pipe under investigation. An internal hydraulic jump was created by attaching a dam-shape barrier on the bottom wall of the pipe. A relation of conjugate depth ratio to the discharge ratio of the two phases was drawn by experimental results. / Ph. D.