We investigate the role of the quartic anharmonicity in the lattice dynamics and thermal transport of the type-I clathrate Ba_8Ga_16Ge_30 based on ab initio self-consistent phonon calculations. We show that the strong quartic anharmonicity of rattling guest atoms causes the hardening of vibrational frequencies of low-lying optical modes and thereby affects calculated lattice thermal conductivities $ąppa$_L significantly, resulting in an improved agreement with experimental results including the deviation from $p̨pa$_L∝T^-1 at high temperature. Moreover, our static simulations with various different cell volumes shows a transition from crystal-like to glasslike $kp̨a$_L around 20 K. Our analyses suggest that the resonance dip of $kaą$_L observed in clathrates with large guest free spaces is attributed mainly to the strong three-phonon scattering of acoustic modes along with the presence of higher-frequency dispersive optical modes.