Abstract
We have obtained CO(J = 2−1) spectra of nine face-on low surface brightness galaxies using the JCMT 15 m telescope and observed H images using the 2.16 m telescope of NAOC. As no CO has been detected, only upper limits on the H2 masses are given. The upper limits of total molecular hydrogen masses are about . Their star-formation rates are mainly lower than yr−1 and star-formation efficiencies are lower than yr−1. Our results show that the absence of molecular gas content is the direct reason for the low star-formation rate. The low star-formation efficiency probably resulted from the low efficiency of H i gas transforming to H2 gas.