The aim of this study was to assess the strains of probiotic bacteria in yoghurt for their ability to produce CLA from free linoleic acid. The effect of process variables was investigated on increasing of CLA in probiotic yoghurt. Two different set of microorganism consists of L. acidophilus, B. lactis and traditional yoghurt starters and in second group L. acidophilus, B. breve was studied as probiotics besides starter culture. Results showed that in both sets, the highest amount of CLA was obtained by addition of 2% non-fat dry milk, the addition of safflower oil (1.4 ml/l milk) in pH=6.0, incubation temperature of 37°C, and termination of incubation at pH=4.5. In the most suitable condition‚ the amount of CLA in probiotic yoghurt containing B. lactis increased by 450% from an average of 0.04 mg/ml in non-treated yoghurt to 0.22 mg/ml fat in the probiotic yoghurt containing safflower oil. But in the case of B. breve, the content of produced CLA was increased 10 fold from 0.04 to 0.4 mg/ml. Anyway, the viability of L. breve was less than L. acidophilus by 2 log CFU/ml. So for further investigation encapsulation of B. breve is suggested to reach to a product with the high amount of CLA and enough viable cells. No significant difference between product and control was observed in the ranking sensory test.