The fitnessof a genetically engineered microorganism strainthat constructed as transconjugant by conjugation mechanism of horizontal gene transfer in Pseudomonas aeruginosa bacteria comparing with its parent strains has been assessed. Ecological fitness expressesinteractionof an organism with its environment. This consider as a good indicator for the assessment of Genetically Engineered Microorganism (GEM). When released into the environment this assay was carried out under laboratory condition to investigate the effects of environmental condition on the fitness. These laboratory condition included the effect of differentcations(Na⁺, Ca⁺², and Fe⁺³), different incubation temperatures, pH values and different ratios of the two parents. Increasing of mono, and di cations (Na⁺, Ca⁺²) concentrations revealed an increasing in the fitness of parents and transconjugant (GEM( strain up to the maximum concentration (105 mM) with Na⁺ and (85mM) with Ca⁺² and then began to decrease.In the three strains temperature plays an important role in gene transfer rate and fitness, There was a direct relationship between the increasing in temperature until it reaches the optimum temperature and the rate of gene transfer. Also, the highest rate of gene transfer via conjugation appeared when the ratio between the donor and the recipient was 3:1.The best results for bacterial fitness for transconjugant strain and parentsappeared at pH 9, which means increased fitness for both of them.These results show that the transconjugant strain can be have and survive well as its parent strain and the effects of some ecological factor are similar in the three strains.