We have obtained CO ( J=2-1 ) spectra of nine face-on low surface brightness galaxies ( LSBGs ) using the JCMT 15-meter telescope and observed H \alpha images using the 2.16-meter telescope of NAOC .
As no CO has been detected , only upper limits on the H _ { 2 } masses are given .
The upper limits of total molecular hydrogen masses are about ( 1.2-82.4 ) \times 10 ^ { 7 } M _ { \odot } .
Their star formation rates are mainly lower than 0.4 M _ { \odot } yr ^ { -1 } and star formation efficiencies are lower than 1.364 \times 10 ^ { -10 } 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 HI gas transforming to H _ { 2 } gas .