Bou Rachana says she no longer feels safe in Cambodia since her husband's death, and wants to move her family to Australia for protection.
She has four young sons and is five months pregnant with a fifth child.
She plans to lodge a formal application for asylum with the Australian embassy in Phnom Penh as soon as she has finished with funeral arrangements and a period of mourning.
He was shot several times in the torso.
But ordinary Cambodians believe his death was a political assassination orchestrated by the Government of Prime Minister Hun Sen.
Kem Ley had been a fearless and vocal critic of the Government and regularly spoke out against corruption and abuse of power.
Just last week he had highlighted a report by an anti-corruption group — Global Witness — which questioned Hun Sen's personal wealth.
The report alleged that Cambodia's ruling family were "pulling the strings on Cambodia's economy and amassing vast personal fortunes with extreme consequences for the population".
It also accused Hun Sen of presiding over a "30-year dictatorial reign of murder, torture and the imprisonment of his political opponents" underpinned by a "huge network of secret deal-making and corruption".