INTRODUCTION The purpose of the present article is to employ an appropriate solution for falling cylinder rheometer to a power law fluid, and then design and construction the F.C.R. to measure the rheological properties of Polyvinylalcohol with different concentrations The study of flow past a rigid object is of great practical importance and because it is the foundation of a branch of fluid mechanics namely particle mechanics has been a subject of many theoretical, numerical and experimental investigations. An extensive literature review can be found in Happel and Brenner [1], which deals with particle motions in a Newtonian fluid at low Reynolds number. The particle mechanics is the theoretical basis of an experimental approach known as the falling object viscometery which consists in allowing an object to fall freely in the fluid. After initial acceleration, when the external drag on the surface and buoyancy become equal to the downward force due to the gravity, the object attains a constant terminal velocity. Falling balls because of their fast and simple operations have been widely used to determine the viscosity of the Newtonian and non-Newtonian fluids [2-5]. The falling cylinder viscometer is getting more favoured than a falling ball viscometer because it is easier to construct cylinder with predesigned densities and is more amenable to analysis than the sphere ball for non-Newtonian fluids. Design and evaluation of the falling cylinder rheometers for non-Newtonian fluids obtained by Lohrenz [6], Lohrenz et al. [7], Ashare et al. [8] and Eichstadt and Swift [9]. Their mathematical investigations were conducted by assuming no end effects, in essence, a cylinder of infinite length. The important in the design of the system of falling viscometers to account for end effects of Newtonian fluids was treated by several investigators [10-13]. THEORY When a cylinder is dropped in a viscous fluid, after initial acceleration, the cylinder attains a constant velocity known as terminal velocity. There are three forces acting on the cylinder, force of gravity (acting downward), buoyancy force (acting upward), and drag force (acting upward). It is assumed that the ratio of the length to the radius of the cylinder (L/R) is sufficiently great which end effects can be neglected. When terminal velocity is reached the Newton’s second law reduces to the requirement that the forces on the cylinder sum algebraically to zero. Etemad et al. [14] obtained the falling equations for a steady, Theoretical and Experimental Study of the Rheological Behaviour of Non-Newtonian Fluids Using Falling Cylinder Rheometer S.Gh. Etemad, R. Bagheri and S.Zeinali. Heris Chemical Engineering Department Isfahan University of Technology 84154 Isfahan, Iran ABSTRACT The present study attempted to solve falling cylinder governing equation for power law model fluids and also construct a novel falling cylinder rheometer (F.C.R.) to measure the rheological properties of power law model non-Newtonian fluids. Different falling cylinders with predesigned densities were used to determine the apparent viscosity of Polyvinylalcohol solutions in water with various concentrations. The results indicate that all P.v.a. solutions obey the power law model which power law index as well as consistency index change linearly with concentration. Increasing concentration of the solution decreases power law index while enhances consistency index and apparent viscosity.