Thermalization in isolated quantum systems is closely connected to the notion of quantum chaos, as suggested by the eigenstate thermalization hypothesis (ETH). Random quantum circuits provide an ideal platform for the study of this connection. In this talk, I will introduce a field theoretical approach to study quantum chaos and thermalization in random quantum circuits. This approach allows us to investigate the statistical properties of quasienergy eigenstates and eigenvalues for a wide class of Floquet random quantum circuits. Our calculation of eigenstate correlation function provides analytical evidence supporting an essential aspect of ETH. Moreover, it indicates that the time-dependent relaxation towards thermal equilibrium is strongly linked to a probe of energy level statistics - the spectral form factor.