Isotope shifts Ds^148,154\Delta \sigma^{148,154} in 200 spectral lines of neutral samarium atom (Sm I) in the region of 355-455 nm (28160-21970 cm^-1) have been measured using a Fourier Transform Spectrometer. Isotope shift data in 187 spectral lines of Sm I are being reported for the first time. Level isotope shifts, DT^148,154\Delta T^{148,154} of 119 high odd- and 7 low even-parity energy levels have been evaluated using present isotope shift data combined with the earlier available data. These DT^148,154\Delta T^{148,154} values have enabled us to check the assignments to 4f ⁶6s6p and 4f ⁵5d6s ² configurations of the odd-parity levels made earlier on the basis of parametric calculations. For several levels, the reported compositions of the mixed configuration of type [4f ⁵(⁶H⁰, ⁶F⁰) 5d6s ² + 4f ⁶(⁷F) 6s6p(³P⁰)] have been interpreted with the help of our DT^148,154\Delta T^{148,154} values. 20 unassigned energy levels have been uniquely assigned to 4f ⁵5d6s ² configuration. The present study provides DT^148,154\Delta T^{148,154} values of the levels, which were tentatively assigned to 4f ⁵5d ²6s and 4f ⁶5d6p configurations. We could also identify 4f ⁶5d6p configuration for 12 unassigned levels.