Three groups of Carbon Nano Tubes (CNTs) of three different diameters, and each with five different weight concentrations were dispersed in a polyacrylonitrile (PAN)/ dimethylformamide (DMF) polymer solution. Electrospinning technique has been utilized to produce nanofibril composite fabrics from PAN/DMF with CNTs dispersions . Then, the collected fabrics were thermally stabilized and carbonized under static pressure to activate the high surface energy aiming to build firm and strong fabrics. The resultant carbonized sixteen fabrics showed reddish color with strong (as maximum as 60MPa strength) and flexible (as maximum as 5% strain) behaviors. Morphological characterization by using SEM showed minimum average fibril diameter of 130±20 nm for the smallest CNTs diameter. The effect of CNTs size on the electrospun fiber diameter has been investigated. Also, evaluation of the presence of CNTs inside the carbon nano fiber has been investigated by using HRTEM. Mechanical properties (strength and modulus of elasticity) of the fabrics have been investigated. AFM has been used to measure the modulus of single nano fibril composite. Thermal conductivity has been measured precisely to study the effect of CNTs on thermal properties. Positron annihilation lifetime spectroscopy (PALS) was employed to measure the free volume properties of the fabrics and correlated it to fabric's morphological, mechanical and thermal properties. It has been noticed that in general the presence of CNTs decreased the free volume values resulted in dense structure and improved its thermal conductivity. Also, as CNTs diameter decreases the free volume decreases resulted in improvement in fabric's strength and thermal conductivity.