Perovskite oxide thin films and heterojunctions have been fabricated by laser molecular beam epitaxy controlled in atomic scale. Measurements of atomic force microscopy (AFM) and high-resolution transmission electron microscopy (HRTEM) reveal that the surfaces and interfaces of the thin films and heterojunctions are atom-level-smooth. The optically transparent oxide thin films with transmittances higher than 85% in the visible region have been fabricated. The electrical and optical properties of BaTiO3-x thin films and BaTiO3/SrTiO3 (BTO/STO) superlattices were examined. The BaTiO3-x thin films have the properties of insulator, semiconductor or conductor with different oxygen content. The coefficient of the second harmonic generation of BTO/STO is 23 times larger than that of bulk BaTiO3 crystal. We observed the current and voltage modulations of La0.9Sr0.1MnO3/SrNb0.01Ti0.99O3 p-n junctions by applied magnetic fields and found that the p-n junctions exhibited the positive colossal magnetoresistance (CMR) behavior for the first time.The CMR ratio (△R/R0,△R=RH-R0) are 46.7% and 83.4% in 5 Oe,and 1000 Oe at 255 K for LSMO/SNTO p-n junction;and the positive CMR ratio as large as 515% at 100 K at 3T in LSMO/SNTO multilayer p-n heterostructure are found.