Mr Vidal said he shared McVeigh's horror at the former US Attorney General who ordered the storming of a cult stronghold in Waco, Texas, in 1993, resulting in the death of more than 80 cult members.
McVeigh invited Mr Vidal to be present at the 16 May execution after the two corresponded for more than two years.
Mr Vidal said that he shared McVeigh's feeling that government had "run amok".
He said he hoped his presence at the execution, despite his opposition to the death penalty, would be seen as a sign that "there is somebody who's not violent - that's me - who agrees" with McVeigh, Mr Vidal said on US television.
Common concerns
He said there was a "general pattern of harrassment of American people across the country" by the US Government.
Mr Vidal said he shared McVeigh's horror at the former US Attorney General who ordered the storming of a cult stronghold in Waco, Texas, in 1993, resulting in the death of more than 80 cult members.
"I'm against the death penalty, I'm against Timothy McVeigh blowing up people in Oklahoma City, but I'm even more against [former] Attorney General Janet Reno," Mr Vidal said.
The writer and political commentator is planning to write about the execution for Vanity Fair magazine.
In his opposition to the death penalty, Mr Vidal is out of step with the majority of Americans.
Four out of five Americans support the execution of Timothy McVeigh, according to a poll for the Associated Press news agency.
The same poll suggested that 28% of people opposed to the death penalty support it in McVeigh's case.
Broadcast
About half of respondents said families of the victims should be allowed to watch the execution on closed-circuit television.
They will be allowed to do so.
A private company, Entertainment Network Incorporated, has sued to be allowed to broadcast the execution on the internet, but a federal judge ruled against the company on 19 April.
There have been fears that the closed-circuit broadcast will somehow be intercepted and released on the internet despite the ruling.
The US Government is working to ensure that the broadcast is secure.