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Shape tracking using a fiber Bragg grating sensor is a promising tool due to its thin, flexible, and weightless nature. Conventional investigations attached optical fibers with a metal rod which limited the curvature due to its stiffness and increased distance between the center of the fiber and that of the sensor. We fabricated ultrathin flexible shape sensor with 110 mm length and 560 �� PGLDPHWHU��,WVORZ�cannula. The sensor causes only miniscule resistance when the cannula rotates or moves, and allows extra spaces for other surgical tools thanks to its small diameter. After the calibration, the sensor was integrated to the active cannula and overall position of the sensor was tracked. It showed an average error of 3.17 ± 1.21 % of the lengths of the measured nodes from the fixed starting point. The error was comparable to the outer diameter of the cannula, 2.8mm.