The invention relates to a method for the synthesis of diacids of general formula ROOC—(CH2)n—(CH═CH)a—(CH2)mCOOR1 in which n and m are identical or different and each represent an integer such that their sum is between 6 and 15, a is an index of 0 or 1, and R and R1 are H or an alkyl radical with 1 to 4 carbon atoms, from long-chain mono-unsaturated natural fatty acids or esters having at least 10 adjacent carbon atoms per molecule of the general formula CH3—(CH2)p—CH═CH—(CH2)q—COOR, p and q, being identical or different and representing indices between 2 and 11, wherein said method comprises the first step of oxidizing by fermentation said natural fatty acid or ester, using a microorganism, such as a bacterium, a yeast, or a fungus, into at least one unsaturated dicarboxylic acid or dicarboxylate, the second step of submitting the product from the first step to a metathesis crossed with a compound of formula R2OOC—(CH2)x—CH═CH—R3, in which R2 is H or an alkyl radical with 1 to 4 carbon atoms, x is 0 or 1 or 2, and R3 is H, CH3 or COOR2, in order to obtain the unsaturated compound of formula ROOC—(CH2)q—CH═CH—(CH2)x—COOR2, and the third optional step of converting the unsaturated compound into a saturated compound by hydrogenation of the double bond.