The most famous of the speeches from Martin Luther King, Jnr is the "I have a dream" speech, delivered at the March on Washington in 1963. Best heard spoken, it demonstrates the power King had with the spoken word.
He uses a number of rhetorical devices over the course of the speech.
He repeats key words or phrases. These resonate with the listener, and each one re-enforces the last. This repetition for emphasis leaves the hearer with ideas and phrases they will easily remember.
"I have a dream..." repeats many times, each time reinforcing his vision of the future, a dream of equality.
King delivered his speech 100 years after the Emancipation Proclamation, on the footsteps of the Lincoln Memorial, the president who freed the slaves in America. This was alluded to in his speech, underlining the need for continuing the move forward for the civil rights movement.
"We hold these truths to be self evident: that all men are created equal."
He uses metaphors to colour his language and help create a picture in the mind of the listener. His metaphors are cleverly well crafted, and powerful. Below the metaphor of the mountain of despair indicates the uphill struggle of black Americans seeking civil rights, and like a prospector, will dig through the rock to reach the gem of liberty as a reward for the struggle at hand.
"We will be able to hew out of the mountain of despair a stone of hope."
As far as spoken rhetoric, King, a passionate preacher, used many vocal techniques to reinforce his ideas. The rhythm of his language creates an expectation of when the next key point will occur. He pauses to great effect, particularly after saying "I have a dream" forcing the audience to listen and think about what he has just said. He alters his pace and volume, leading to crescendos at key moments, then pausing for applause, all clearly audible in the video of the speech.
His parallel phrasing shown below create rhythm, and allow for building up vocally to create a spectacle for the crowds.
"We will be able to work together, to pray together, to struggle together, to go to jail together, to stand up for freedom together…"
For more analysis of Martin Luther King's speech, check out this interesting video:
You might want to clarify on the what MLK speech you are referring to. I'm going to assume that it's the "I have a dream speech" since it is commonly referred to. Just to clarify, a rhetorical device is a technique that conveys meaning/evokes emotion/persuades the audience.
here are some examples of rhetorical devices that Martin Luther King uses:
ex: "I have a dream that one day this nation will rise up...I have a dream that my four little children..." etc.
ex: "one hundred years later..."
ex: "their destiny is tied up with our destiny"
ex: "dark and desolate"