The Duke of Cambridge waas also ful of praise for the portrait.
Prince William said:: "It's beautiful, it's absolutely beautiful."
The Cambridges later attended a private breakfast reception at the London gallery to mark the unveiling.
The first official portrait of the Duchess of Cambridge was unveiled today. The Duchess and her husband the Duke had a private viewing of the painting this morning.
It goes on display to the public this afternoon. The royal couple spent around 10 minutes looking at the portrait privately then met Emsley and his wife Susanne and daughter Marie.
The first official portrait of the Duchess of Cambridge's has been unveiled.
The Duchess is reported to have said the painting was "amazing".
It was painted by Paul Emsley and was commissioned by the National Portrait Gallery.
The Duke and Duchess of Cambridge are expecting a baby.
St James's Palace said the couple are very pleased with the news.
The Duchess of Cambridge has arrived at the Treehouse Hospice in Ipswich where she will give her first public speech since becoming a royal. She was greeted by crowds of children who'd waited all morning for her to arrive.
Kate will be shown around the hospice and meet children and their families who receive care and support from East Anglia's Children's Hospices (EACH), before making her first public address at about 11.30am.