Abstract: The designs of the two cold neutron guides, CNG1 adn CNG2, to be built in China advanced research reactor (CARR) are studied with MonteCarlo simulation technique. The neutron flux density at the exit of the both guides can reach above 1×109 cm-2·s-1 under the assumed flux spectrum of the cold neutron source. The transmission efficiency is 50% and 42%, and the maximum divergence is about 2.2° and 1.9°, respectively for CNG1 and CNG2. Neutron distribution along horizontal direction is quite uniformal for both guides, with maximum fluctuation of less than 3%. Gravity can affect neutron distribution along vertical direction considerably.