Polymer blends is a subject of great importance for academic as well as industrial interest. The objective of this research programme is to study the miscibility of polymer blends. Polymer blends in general are of three types: amorphous-amorphous; amorphouscrystalline; and crystalline-crystalline systems. This work is concerned with crystalline-crystalline type polymer blends and is restricted to polyolefin crystalline polymers only. Firstly an outline of crystalline-crystalline polymer blends mainly concentrating on polyolefin blends has been given in an introductory chapter. This chapter presents relevant fundamentals rather than being an extensive critical review of the literatures. In the first phase of the work three polyolefins (low-density polyethylene, high-density polyethylene and linear low-density polyethylene) have been melt mixed to prepare three sets of binary blends covering the entire range of compositions. A variety of experimental techniques were used to assess the miscibility of the blends prepared. The whole experimental programme can be subdivided into three major areas: the first is concerned with structural characterisation; the second area with the melt properties; and the third with the mechanical properties of the polymers in the solid state. Polymers (including their blends) and experimental techniques are detailed in Chapter 2. Experimental results are discussed and concluded individually in four subsequent chapters. Chapter 3 gives mainly the information of molecular structure of the three polyolefins. Characterisation of crystalline structure of polyolefins and their mixtures has been covered in Chapter 4. Chapter 5 is concerned with melt properties of polyolefins and their blends; whereas Chapter 6 deals with solid state mechanical properties. In these last two chapters an attempt has been made to correlate, where possible, the structures with their properties. From the experimental investigations it has been found that the system of linear low-density and high-density polyethylene blends is miscible whereas the system of low-density and high-density polyethylene is not. Structural characterisation has shown that the low-density and linear low-density polyethylene blends system is immiscible at all compositions, although some mechanical,compatibility has been found from melt property and mechanical property measurements. The second phase of the work was mainly concerned with the application of polyolefin blends. Film has been manufactured from polyolefin blends (including their components) and evaluated. Linear low density/ high-density and low-density/linear low-density polyethylene systems have been selected for this part of the work (Chapter 7). Essential to the understanding of the blends involving linear low-density polyethylene, is appreciation of the molecular structure of the homopolymer. A preliminary characterisation of linear lowdensity polyethylene has revealed that its structure is more complex than might have been foreseen. Finally in Chapter 8 overall concluding remarks have been made based on the conclusions drawn from all investigations carried out in this research programme and reported in the individual chapters.