In this thesis the galactic cosmic ray spectrum during Forbush decreases was studied by using daily cosmic ray data of the neutron monitor stations obtained from the High resolution neutron monitor database (NMDB). Following the method of the coupling coefficients as considered by Wawrzynczak and Alania (2010), the cosmic ray intensity in the heliosphere and the spectral index were calculated for different Forbush decreases. Also the solar, interplanetary and geomagnetic activity which associated by the selected episode of Forbush decreases were studied analytically. In this work according to Wawrzynczak and Alania (2010) the daily galactic cosmic ray intensity in the heliosphere during the Forbush decreases was calculated for various values of spectral index (γ) in a particular range (0.5-2 with step 0.01) by using the program of Matlab. Αn acceptable γ has to be corresponded to the values of the cosmic ray intensity in the heliosphere that are almost the same for all neutron monitors due to the fact that they are independent of topical characteristics. For the calculations of spectral index three different coupling functions were applied in order to find out the most appropriate coupling function for the calculation of the cosmic ray spectrum. Firstly the total function of Clem and Dorman (2000), secondly the function of Belov and Struminsky (1997) including a separate term E^3.17 extending the first for low kinetic energies to the neutron monitors with rigidity between 1GV < Rc< 2.78 GV, were found to be suitable for the study of Forbush decreases. Finally, the recently established yield function of Mishev et al. (2013) was applied first time for Forbush decreases. The results and the calculations of this thesis for the cosmic ray energy spectrum are very important for Space Weather applications. ; Στην παρούσα διατριβή μελετήθηκε το ενεργειακό φάσμα της γαλαξιακής κοσμικής ακτινοβολίας κατά την διάρκεια μειώσεων Forbush χρησιμοποιώντας δεδομένα από την Υψηλής Ανάλυσης Βάση Μετρητών Νετρονίων (NMDB). ...