In the beginning of the play it is Lady Macbeth who has power over Macbeth, but as the play develops, the power shifts to Macbeth.
The first demonstration of Lady Macbeth's power is revealed in her soliloquy in Act I, During this soliloquy she states
"Hie thee hither
That I may pour my spirits in thine ear
And chastise with the valor of my tongue
All that impedes thee from the golden round."
The "golden round" is the crown. She knows that Macbeth is a "push over" because as she says, Yet do I fear thy nature; / It is too full o' th' milk of human kindness." But it is quite obvious that she feels that she is more powerful than Macbeth early in the play, for once Macbeth comes home, she does pour her spirits into his ear.
When Macbeth comes home and tells her that the King is coming to their dwelling to celebrate, she asks when will he be leaving, and when Macbeth states he plans to leave in the morning, Lady Macbeth responds by saying
Shall sun that morrow see!
meaning King Duncan will not live to see the sun rise. This stuns Macbeth, but he does not respond, well at least not verbally. His expression does though, and Lady Macbeth demonstrates her power and lets him know by saying
Your face, my thane, is as a book where men
May read strange matters. To beguile the time,
Look like the time. Bear welcome in your eye,
Your hand, your tongue. Look like th' innocent flower,
But be the serpent under 't. He that's coming
Must be provided for; and you shall put
This night's great business into my dispatch,
Which shall to all our nights and days to come
Give solely sovereign sway and masterdom.
In these lines, she most certainly reveals her power over Macbeth, by telling him to put tonight's "great business into [her] dispatch" and if he does, she states that it will give them "sovereign sway and masterdom."
Macbeth's response to this is "We will speak further." Lady Macbeth exercises her power once again over Macbeth and says,
Only look up clear.
To alter favor ever is to fear.
Leave all the rest to me.
The line truly reveals her power over Macbeth, for he does just that.
Later in Act I when Macbeth convinces himself to not murder King Duncan, Lady Macbeth once again reveals her power by questioning his manhood by saying, that when you first wanted to do this, "then you were a man."
And then Macbeth asks, "If we should fail?" And Lady Macbeth flexes her power once again, and says, "But screw your courage to the sticking-place, / And we'll not fail." Then she lays out the plot to murder Duncan, and after hearing her plot, Macbeth says that he is "settled, and bend up / Each corporal agent to this terrible feat."
Once the murder is complete, and they have been crowned king and queen, the power begins to shift. For Macbeth does not consult with Lady Macbeth at all, and plots the murder of Banquo and his son and Macduff's family. As a matter of fact, Lady Macbeth's role becomes almost insignificant after Act III.