Abstract: The present study aims to determine different biological aspects of the new laelapidae mite Cosmolaelaps qassimensis which was collected from Qassim area in (2013-2014) and fed on the two acarid mites, Tyrophagus putrescentiae (Schrank) and Caloglyphus rodriguez Samsinak and compared with the egg masses of root-knot nematode, Meloidogyne incognita chitwood as food sources under laboratory conditions of 26±1°C and 70±5% RH. The incubation period of male and female eggs averaged 2.7 and 2.99 days, respectively. Female life cycle was significantly shorter when it fed on T. putrescentiae followed by C. rodriguez and then egg masses of M. incognita. Life span of female was sub-equal, when C. qassimensis fed on either T. putrescentiae or C. rodriguez and significantly shorter, than those on M. incognita. Male life cycle, longevity and life span were significantly shorter when C. qassimensis fed on T. putrescentiae followed by C. rodriguez and egg masses of nematode. The total number of eggs per female feeding on T. putrescentiae was significantly higher than on C. rodriguez and followed by M. incognita. Mean generation Time (T) and the time required for the population of C. qassimensis to double its number (DT) were shorter when it fed T. putrescentiae than on C. rodriguez and M. incognita. Net reproductive rate (Ro), the Gross Reproductive Rate (GRR), intrinsic rate of increase (rm) and finite rate of increase (erm) were higher, when C. qassimensis fed T. putrescentiae and C. rodriguez while, these parameters were at their lowest rates, when C. qassimensis fed egg masses of M. incognita. Tyrophagus putrescentiae proved to be the most suitable food source to obtain the largest numbers of offspring of the predatory laelapidae mite C. qassimensis.