This article presents an endfire leaky wave antenna (LWA) by periodically loading a series of strips and stubs between and at two sides of the coplanar strip (CPS) line to modify its phase and attenuation constants separately. The inner loaded strips are mainly utilized to generate a leakage region for the CPS to acquire the endfire frequency point as the normalized phase constant equals to 1. The outer stubs at the corresponding positions of inner ones play the role to control the attenuation constant of the antenna. It shows that the proposed structure can realize different attenuation constants with almost the same phase constant values and slopes in the endfire frequency range by just modifying the inner strip and outer stub lengths. The mechanism of the antenna is first discussed to explore its radiation properties under different phase constants. Based on the equivalent circuit model of simplified element, the propagation characteristics are then analyzed to evaluate the effects of different circuit values on the endfire operation point. After that, the propagation constants of designed structure are further discussed under various situations by employing short-open-load (SOL) method in full-wave simulation to discover the realizable attenuation values. Finally, a horizontally polarized prototype with Taylor distribution is designed to achieve -20 dB sidelobe level to verify the effectiveness of the proposed loading strategy on flexible modification of the propagation constants, where the simulated results agree well with the measured ones and the main beams are not disturbed in the endfire frequency range.