Within the framework of the systematic functional grammar (SFG), Matthiessen (2004) has provided an analysis of the word/element order according to which word/element order in a clause is decided by experiential, interpersonal and textual metafunctions. In this study which has been conducted aiming at the description and analysis of the word/element order in a simple Persian clause, we have sought to describe and analyze the sequence of the main elements in a Persian active simple declarative clause within Matthiessen (2004)'s metafunctional model. In fact, through evidence from Persian language, i.e. samples derived from Persian texts and also other Persian researchers' works, we have sought to demonstrate how the three experiential, interpersonal and textual metafunctions are involved in determining the order of the main elements in an active simple declarative clause in Persian. The results of this study show that the "basic" order of the main elements in a Persian simple clause and specifically single-Complement clause is in line with both experiential and interpersonal metafunctions and any type of the movement of the aforementioned elements in an active simple declarative clause in Persian language is explained with the help of the mechanisms of textual metafunction.