The most suitable structure of the ultrathin silica based imagefiber for the medical usage were investigated by evaluating the image transmission characteristics and the mechanical characteristics. As a result, following facts have been clarified, 1) In the case of NA 0.43, the suitable structural parameters are core-cladding ratio 1.5 and pixel diameter 5 μm. 2) The 0.27 mm diameter imagefiber can stand more than 103 times repeated bending in 10 mm radius.