An optothermomechanical (OTM) device was designed and constructed for fiber characterization with a double-beam interference microscope. This device enabled us to correlate both the mechanical properties and the thermal properties with the optical properties of fibers. The OTM device consisted of three parts, which were used for the drawing (stress-strain), cooling, and heating of the fibers, The designed OTM device (cooling and heating) was attached to the Pluta microscope for the determination of the optical properties of high-density polyethylene (PE) fibers at different temperatures (0-50°C). Also, this OTM device (drawing and heating), connected to the Pluta microscope, was used to study the influence of the temperature (1050°C) and draw ratio (1-7) simultaneously on the optical properties of polypropylene (PP) fibers. Microinterferograms were provided for illustration.