We revisit the vertical structure of black hole accretion disks in spherical coordinates. By comparing the advective cooling with the viscous heating, we show that advection‐dominated disks are geometrically thick, i.e., with the half‐opening angle Δθ>2π/5, rather than slim as supposed previously in the literature. We suggest that such advection‐dominated thick disks around stellar mass black holes may account for the brightest ultraluminous X‐ray sources.