The main goal of this thesis is to explain Gaius Marius military reforms which were implemented at the end of 2nd century BC. To fully understand Marius's military reforms, adequate knowledge about the time that preceded Marius's reforms is needed. Therefore, it is necessary to be familiarized with development of Roman army from the age of kings up to the late Republic. Hence, this thesis shortly analyzes military reforms that were carried out since the founding of Rome up to Marius. Also, thesis analyzes social and political situation in the Republic with emphasis on the second half of the 2nd century BC and development of latifundia. At the end, author analyzes in greater detail circumstances in which Marius's reforms were implemented. Each paragrpah explains a part of the reform which consists of: recruiting citizens no matter the cenzus, reorganization of legion, discipline and training, implementing the eagle as the only simbol of legion and modification of spear.