Structure formation during the isothermal crystallization of a linear polyethylene ( M-w = 32 100 g/mol, M-w/M-n = 1.1) has been monitored by simultaneous real time small-angle x-ray scattering (SAXS) and wide angle x-ray scattering (WAXS) with synchrotron radiation. Changes in the crystalline and amorphous scattering in WAXS occur simultaneously with the changes observed in the SAXS intensity profile, suggesting that the resulting scattering is a consequence of forming crystals. At the early stage of crystal growth, the SAXS intensity at q<0.02 Angstrom(-1) increases while that at higher wave vectors remains constant. Meanwhile, the apparent crystallinity from WAXS increases to about 10% before two peaks in the SAXS intensity profile can be resolved. As lamellar stacks develop, the period corresponding to the first SAXS peak, L-1, decreases initially. After reaching a minimum, L-1 further increases with crystallization time. On the other hand, the period corresponding to the second SAXS peak increases with crystallization time. At late times, the rate of increase is very slow. Changes in the crystallinity obtained from WAXS and the total scattering power from SAXS were evaluated and compared with a model for the crystallizing system which accounts for changes in the fraction of lamellar aggregates. The relative importance of two secondary crystallization processes, namely the formation of new lamellar stacks and increase in the crystallinity within lamellae, were evaluated during and after primary crystallization.