The longitudinal spin Seebeck effect (LSSE) with a vertical temperature gradient is one of the most important mechanisms to generate pure spin current. Previous studies of the LSSE excited spin current focus mainly on the magnetic insulators, a little on ferromagnetic metals, and rarely on ferromagnetic half metals. In this work, we demonstrate a significant spin current injected from the highly spin polarized ferromagnetic half metal La0.7Sr0.3MnO3 by the LSSE. The sign of the thermal voltage can be reversed by using the spin current detector Cr with a large negative spin Hall angle. The ratio of the inverse spin Hall voltage to the total thermal signal in La0.7Sr0.3MnO3 is much larger than that in ferromagnetic metals, such as permalloy and CoFeB. The nontrivial temperature-dependent voltage suggests that the thermal transport in La0.7Sr0.3MnO3 is carried by magnons. This study provides insight into the mechanism of thermally excited spin current in ferromagnetic half metals and recommends the highly spin polarized La0.7Sr0.3MnO3 as a promising candidate for metal-based spin caloritronics devices.