Nanoscale particle size distribution can be inverted by truncated singular value decomposition (TSVD) method. However, it is difficult to select the optimal truncated parameter. Based on the analyzation of TSVD method, we present a nonnegative iterative truncated singular value decomposition (NNI-TSVD) method for obtaining the particle size distribution of nanoscale particle suspensions from dynamic light scattering data. Furthermore, we modify the L-curve criterion for choosing the optimal truncated parameter.Experimental data show that with the NNI-TSVD method, its optimal truncated parameter selected by the second truncated L-curve criterion, can be employed to accurately get the average size and size distribution of unimodal suspensions. The relative error of the inverted average diameter is less than 3%.