In this paper a novel approach to localization of a linear structure object (spine) in medical images using one-dimensional Hidden Markov Model is proposed. Feature sequence of a linear structure object is extracted by using a horizontal-line sampling window with fixed width along the central axis of the object in training phase and the model training is performed through maximum likelihood estimation provided by the Baum-Welch algorithm. A specific localization method derived from Viterbi algorithm is presented for determining a feature sequence in a test image, from which the position of a linear structure object could be obtained. The use of heuristic information improves obviously the performance of localization and the computational complexity. The experiments demonstrate the effectiveness in localization application based on the simple feature expression and the proposed localization method.