We present the first X-ray Luminosity Function ( XLF ) for an optically-selected sample of 49 nearby poor clusters of galaxies and a sample of 67 Abell clusters with z \leq 0.15 .
We have extended the measured cluster XLF by more than a factor of 10 in X-ray luminosity .
Our poor cluster sample was drawn from an optical catalog of groups with 0.01 \leq z \leq 0.03 composed of Zwicky galaxies .
The X-ray emission was measured from the ROSAT all-sky survey .
About 45 % of the poor clusters were detected with 0.5-2.0 keV luminosities from ( 1.7 - 65 ) \times 10 ^ { 41 } h ^ { -2 } ergs/sec .
These are among the X-ray brightest , optically-selected poor clusters in the northern hemisphere .
For this sample , the poor cluster XLF was found to be a smooth extrapolation of the rich cluster XLF .
A new Hydro/N-body simulation of a Hot + Cold dark matter model with \Omega _ { total } =1 , \Omega _ { \nu } =0.2 , and a baryon fraction of 7.5 % was used to model and understand our observational selection effects .
We found that the observed cluster Gas Mass Function was consistent with our model .