The integration of variable renewable energy (VRE) in the power system is an essential way towards net-zero carbon emission. However, the increasing penetration of VRE deteriorates the frequency support capabilities of power system, while introducing challenges for power transmission. This paper proposes a novel frequency constrained co-planning model of VRE stations and transmission lines. The mathematical formulations of frequency constraints and transmission network expansion planning (TNEP) are first introduced to examine the frequency stability and find out the most economical planning scheme of transmission lines under certain capacities of VRE stations, respectively. Then the sensitivity analysis on the capacity of each VRE station is conducted, thus given a co-planning scheme that realizes a tradeoff between energy efficiency and economy. Case studies on the IEEE 14-bus system validate the effectiveness of the proposed method.