The present invention disclosed a multilayer core optical fiber, ML-COF to improve the radiation dose sensitivity for low dose dosimetry applications consist of multilayer fiber, characterized in that the layers consist of doped layers separated by layers with different materials or different concentrations of dopant element with a solid center core or a hollow in the center. The ML-COF is very sensitive compared to a conventional fiber with single core layer. The ML-COF structure would be very useful for designing high-sensitive radiation dosimeter for low dose to medium doses and in-vivo medical applications for diagnostics and radiotherapy applications.