The present work describes a research approach to the aerobic bioleaching of U (IV) ores. Three strains (Acidithiobacillus sp.) isolated from sulfur springs in Ramsar province, Iran and nucleotide sequences of gene fragments of examined Acidithiobacillus sp. strains were recorded in NCBI .Then, uranium bioleaching experiment carried out with native bacteria in different pulp densities. In following, the performance of the isolates was improved progressively by optimizing the pH and initial iron concentrations. The results showed that total uranium extraction was achieved with 100% efficiency using native isolated Acidithiobacillus sp. strain FJ2 in 2.5, 5, 7.5 and 10% pulp densities of the ore. Whereas, the isolates could leach the U (VI) with 68% efficiency at 12.5% ore pulp density. In addition, Acidithiobacillus sp. strain FJ1 leaded to 100% uranium extraction at 2.5% pulp density and 59%at 5% pulp density of the ore. As well, determined optimized values showed that initial ferrous iron concentration at 2.5 g/L and initial pH at 2, gives maximum uranium extraction yields. This is very promising results since, the native isolated strains especially Acidithiobacillus sp. strain FJ2 can be used as a capable bacteria in uranium extraction process.