Embodiments of the present disclosure relate to a method and device for determining carriers to be monitored. In example embodiments, a first number of carriers of a first radio access technology (RAT) to be monitored by a terminal device when operating using a second RAT is determined. The second RAT is different from the first RAT. A first total number of carriers to be monitored by the terminal device when operating using the second RAT is determined at least in part based on the first number. A second number of carriers of the second RAT to be monitored by the terminal device when operating using the first RAT is determined. A second total number of carriers to be monitored by the terminal device when operating using the first RAT is determined at least in part based on the second number. Further, when the terminal device operates using both the first and second RATs, carriers to be monitored by the terminal device are determined based on a difference between a first sum of the first and second total numbers and a second sum of the first and second numbers. In this way, terminal inter-RAT mobility may be enhanced. Meanwhile, due to the subtraction of the duplicated numbers, terminal operations may be simplified, and terminal performance may be improved.