We present our QCD analysis of the diffractive pomeron structure function. We consider Pomeron as an object with parton distribution function, evolving according to the NLO DGLAP equations within the framework of the ‘Fixed Flavour Number Scheme’. A remarkable feature of DDIS is the dominance of the twist-4 contribution for small diffractive masses. We quantify this effect and find a reasonable good agreement between the predictions and the data. We predict longitudinal and charm proton diffractive structure function as well. Our results are compared with other analysis from the literature.