An image guide has been made by using polymer optical fibers. Single-strand polymer optical fibers were laid out first in a square array, and the square bundle thus made was fused in a vacuum oven at various processing temperatures. Although the line resolution of an image guide is determined by the number of optical fibers per unit area (i.e., the pixel density), the fusion temperature affects the cohesiveness of neighboring fibers that influences the portrait quality of the image guide. The objective of this study was to investigate the influence of fusion temperature on the portrait quality of the image guide. The image transfer characteristics were evaluated quantitatively by using a computer program that was developed in our laboratory for the comparison with a macrography. The present result indicates that there exists an optimum fusion temperature for the optimum cohesion between neighboring fibers that maximizes the brightness of an image without cross-talk.