The objective of this research was to construct and to evaluate a portable fiber tester (PFT) for in field fleece and fiber test use. The standard wool top (SWT) samples were used for comparative measurement of fiber diameter using PFT in three laboratories without restricted ambient temperature or relative humidity control. Linear model with three factors was used to derive the least square means and variance components were determined. The PFT is constructed to be a compact and lightweight device of high-speed fiber testing. It works using digital image technology that measure and evaluate the fiber diameter variability of each wool sample within 45 seconds. The results show that the average fiber diameters measured by PFT at three labs are within IWTO defined tolerance values. There were no significant lab effect whereas, variance of labs remained as low. It can be concluded that PFT is a new instrument with high precision and accuracy to measure the fiber diameter of wool fleeces, which may be operated in differing field conditions.