The structural features of nanoscale thin films composed of a miktoarm (polystyrene)2-(polyisoprene)2 (PS2-PI2) star polymer were studied for the first time using synchrotron grazing incidence X-ray scattering (GIXS) and X-ray reflectivity (XR) analyses. The acetone-annealed thin films formed a horizontally oriented lamellar structure via phase separation. The lamellar structure displayed interesting and unique features not present in the corresponding diblock copolymer thin films. The thick interfacial layers formed by the PS and PI arm components were three times the thicknesses of the corresponding layers in the diblock copolymer films. The interfacial layers further displayed a relatively high degree of thermal expansion. The overall lamellar structure was thermally stable up to temperatures approaching the degradation temperature. These unique structural features could be understood in terms of the rigidity and bulkiness of the confined crowded environment surrounding the arm-jointer.