To study some physical properties of comets of the Jupiter Family, we have compiled a new catalog of nuclear magnitudes. As a source for the catalog we have used Kamel's (1992) compilation of data from many different sources, adding measurements of nuclear magnitudes for the period: mid 1990 - mid 1996, taken from the IAU Circulars, MPCs, ICQs and our own data. Among the few systematic observers of cometary nuclear magnitudes we can mention E. Roemer's long-term program (60s-70s), which provided nuclear magnitudes for a large sample of JF comets measured on photographic plates. During the last decade the data set of nuclear magnitudes has been greatly enlarged and improved by using CCD detectors attached to medium- and large-size telescopes. Most of the recent published measurements of nuclear magnitudes are from J. Scotti who uses a CCD camera attached to the Spacewatch Telescope. We find a tendency of the measured nuclear magnitudes to be fainter as JF comets are observed with CCD detectors. However, a few JF comets observed vey far from the Sun (4-7 AU) showed brighter nuclear magnitudes than when they were observed closer to the Sun. From the nuclear magnitudes, we have estimated the sizes of the nuclei. Correlating the sizes and the water production rate we estimate the fraction of active area.