This communication presents a compact endfire leaky wave antenna (LWA) presented on the coplanar stripline (CPS) with narrow width property under a new operation mechanism. By periodically loading stubs between two strip lines in parallel, the leaky region of its fundamental mode can be generated in the CPS with forward beam scanning property, and the endfire radiation is realized at the frequency where its normalized phase constant is equal to 1. Since the leaky region of the fundamental mode owns a relatively low cutoff frequency, the proposed antenna generally has the property of small overall size in this region. The endfire operating frequency is first discussed based on the circuit model, and the corresponding radiation property is then analyzed from the antenna array point of view. The leakage constant as well as characteristic impedance of the proposed antenna are further investigated with the full-wave extraction method. Moreover, a modified structure is proposed as the last element of the antenna to leak out the rest of the energy at its end. Finally, a CPS leaky-wave prototype antenna is designed and fabricated, where the measured results show good agreement with the simulated ones and thus validate the predicted performances.