The aim of this master thesis was to calculate the molar mass and the molar mass distribution using rheological data of 13 different semi-crystalline polymers. The measurements were taken with a rheometer to detect the loss modulus, the storage modulus and the viscosity of each sample. The calculation was figured out in two steps to get the molar mass distribution with the use of the program Rheoplus. At first, the continuous relaxation time spectrum was calculated from the data of the storage and the loss modulus. A polypropylene from the demo file of the program Rheoplus was used as reference material for the calculated data. Secondly, the molar mass distribution was figured out and compared with the data from SEC analysis. The comparison between the calculated results and the reference material from the demo file showed that the characteristics of the resulting curves were different in their outer areas, but similar in their inner parts. Furthermore, there was a huge difference after calculating the second part, the molar mass distribution. Although the peaks of the curves are located at similar positions, there was a big difference in their general characteristic regarding the level and the form of the curves. Moreover, the second part of this thesis was to recalculate the rheological data out of the molar mass distribution. The calculation was working in two steps either. The first part was to figure out the relaxation time spectrum data from the distribution of molar mass and finally to calculate the storage and loss modulus as well as the viscosity from the spectrum. The first result was again compared with the result from the first spectrum calculated from rheological data. The spectra are showing big differences in their characteristics. The levels of the functions in the graph as well as their distribution were very different to each other. The same effect happened while comparing the calculated storage and loss modulus to the measured ones. An explanation for this phenomenon could be the fact, that the material taken for the investigations was a long chained branched polymer. It should also be considered that a few miscues appeared in the calculation because the used mathematical models were figured out for amorphous polymers. This thesis shows that it is very difficult to calculate the molecular mass distribution of semi-crystalline polymers from reological data at the moment.