Sun followed up his win in the 400 freestyle on the opening day of competition to win the gruelling 30-lap race in a time of 14 minutes 31.02 seconds, slashing more than three seconds off the previous world record of 14:34.14 he set at last year's world championship in Shanghai.
Canada's Ryan Cochrane took the silver medal in 14:39.63 while Tunisia's Oussama Mellouli, the Beijing Olympic champion, collected the bronze in 14:40.31.
South Korea's Park Tae-hwan, silver medallist in the 400, came fourth as the first seven finishers all broke the magical 15-minute barrier.
There was high drama before the start when Sun lost his balance and tumbled into the water after an official aborted the start when someone in the crowd yelled out.
Swimmers receive an automatic disqualification for false starts but Sun was allowed to compete because the official starter had told the swimmers to stand up from their crouching position because of the noise in the crowd.
When the race did get away, Sun regained his composure and went straight to the lead, churning through the water with seemingly effortless ease, and brought the crowd to their feet when he sprinted the last lap looking as fresh as when he began.
Sun has been one of the outstanding swimmers in London, winning two gold and two silver medals.
Just six other men have completed the Olympic 400-1500 double with Russia's Vladimir Salnikov the last to achieve the feat, 32 years ago, in 1980.
Fogg was last to finish with 15:00.76, but will be pleased to have registered the third-fastest time of his career on such a grand stage.
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