The combination of a high-<i>Q</i> optical microcavity and a saturable absorber is expected to enable mode locking between different transverse modes in a microcavity. This work studies saturable absorption in carbon nanotubes (CNTs) on microtoroids. The CNTs are selectively grown on silica microtoroids by chemical vapor deposition after the cobalt catalysis is selectively deposited on the surface. A Raman spectrum measurement showed that the grown CNTs are about 1.0 nm in diameter, and that the device is applicable for use in the 1550 nm band. The saturable absorption by CNTs is investigated with a counter-propagating pump-probe experiment.