The crystal structures of four diamides represented by the general formula CH3-CONH(CH2),nNHCO-CH3 (n = 2-5) were solved by X-ray diffraction in order to establish the conformational preferences of the polymethylene segments. The results are complementary of our previous works on diamides and diketones where the conformation of the dicarbonylic units were established. The results appear useful to study the conformation of related compounds, as polyamides, where the experimental data are scarce. A folding trend of the polymethylene segment, when the number of methylenes is less than 4, can be deduced. The same conclusions were inferred from al, initio quantum mechanical calculations at the MP2/6-31G(d)//HF/6-31(d) level.