The evaluation of the combined effects between the antibiotics against P. aeruginosa is among the efforts to counteract its antimicrobial resistance. This study aimed to evaluate the possible combined effect of colistin or polymyxin-B with carbapenem antibiotic group against clinical isolates of multi-drug resistant P. aeruginosa. Strains were isolated from biological samples of hospitalized and non-hospitalized patients with any type of infection related to this pathogen. Only carbapenem-resistant strains were included in the study. After determining the minimum inhibitory concentration (MIC) of antibiotics against the strains by broth microdilution test, the checkerboard method was used for evaluation of any possible effect of both colistin or polymyxin-B with imipenem or meropenem. Fifty-nine strains of reduced susceptibility to carbapenems were recovered. It was noted that (45.7%) of strains had an imipenem MIC=1024 µg/ml and (28.8%) had a meropenem MIC=512 µg/ml. The checkerboard technique demonstrated that the ten representative strains showed reductions in their minimal inhibitory concentration (MIC) for at least one of the antibiotics in the combinations evaluated. These results from the in vitro evaluation suggest that combinations of colistin and polymyxin-B may significantly reduce the MICs of the carbapenem antibiotics tested.