Colorectal Cancer (CRC) is a chief cause of mortality globally. While genetic causes contribute for a small proportion of risk factors for this problem, environmental risk is significantly crucial. Among environmental factors are changes of gut microbial composition (dysbiosis). Recently, it is believed that gut microbiota symbiosis (normal composition) is a barrier against inflammatory and carcinogenic bowel diseases and many studies have revealed the importance of maintaining this ecosystem and its metabolic functions for inhibiting the colonization of harmful pathogens. Accordingly, microbial dysbiosis (altered composition) might trigger carcinogenesis causing CRC. Also, accumulating evidence suggests that chronic infection and the sequential inflammation (chronic colitis) contribute to tumor initiation and tumor progression, thus, terms such as microbial-associated CRC have recently appeared similar to colitis-associated CRCs. Regarding the proposed carcinogenic role of microbiome in CRC, several studies reported that analysis of gut microbial community diversity/richness based on 16SRNA gene sequencing has shown significantly reduced microbial diversity in feces of colorectal cancer patients than in controls. This advent (16S rRNA gene sequencing) allows for describing the alterations in the microbiota composition, comparing it to the gut microbiota composition of healthy individual to that of patients with diseases. Thus, it seems achievable to use microbial variation markers in non-invasive early diagnosis and/or prognosis of CRC. However, a lot of work is still being required for making a reliable screening model for detection of CRC patients. Below, this review will explain some facts about microbiome as a critical hidden organ in humans, evidence-proven changes in gut microbiome that might induce and promote colorectal carcinogenesis, and the foreseen probability of establishing a microbial marker and /or targets for diagnosis and treatment colorectal cancer.