We established two formulas to predict molecular weight of polyaminoamine dendrimers and their alkylated derivatives, based on the theoretical number of protons at specific positions in the dendrimers and the true value of the integral values of these protons appearing in proton nuclear magnetic resonance spectra. Calculated results indicated that molecular weight of the dendrimers is approximately equal to results from mass spectrometry. Degrees of alkylation were easily calculated for each dendrimer-alkylated derivative. According to the obtained results, we confirm that the use of the proton spectra can be an effective method to predict molecular weight of dendrimers.