Donald Trump Jr, 39, was live-tweeting as the Comey's testimony went along at the Capitol Hill. He responded to every allegations against President Trump either by the fired FBI director or any of the Senators at the Senate Select Committee on Intelligence.
He was posting repeatedly during the closely watched testimony.
"The market has responded to James Comey. Dow is up! Why? Trump was vindicated today," Trump Jr retweeted a tweet of Adam D Brown, social and political commentator.
"Solid point and seems rather convenient for the partisan nonsense we have seen play out for months," Donald Trump Jr said.
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"Washington DC at its finest. This is what America rallied against, but the DC elite don't get it or know any other way," he wrote in another tweet.
Trump Jr, who played a key role in the last year's presidential elections and is now managing his business, has 1.64 million followers, as against more than 31 million of his father.
"Did I miss something or did Comey just say he asked a friend to leak information to the press? Is this a joke?" Trump Jr wrote posing a question to the statement of the former FBI director.
"We all know that there is no way anyone in the swamp would leak something favourable to #potus. Otherwise this whopper would be everywhere!," Donald Trump Jr wrote.
This prompted mockery from Trump Jr.: "So if he were a 'Stronger guy' he might have actually followed procedure & the law? You were the director of the FBI, who are you kidding?"
I hear "I hope nothing happens but you have to do your job very far from any kind of coercion or influence and certainly not obstruction!" he said.
"Knowing my father for 39 years when he "orders or tells" you to do somethingthere is no ambiguity, you will know exactly what he means," he wrote in yet another tweet.
Fired FBI director James Comey has said that it was "very disturbing" that President Trump asked him to stop probing the Russia links of one of his top aide.
Comey was fired on May 9, when Trump expressed his deep frustration with the continuing probe into whether members of his presidential election campaign had tied up with Russian hackers against the billionaire tycoon's rival, Democrat Hillary Clinton.