Epitaxial ferroelectric thin films on single-crystal substrates generally show a preferred domain orientation in one direction over the other in demonstration of a poor polarization retention.This behavior will affect their application in nonvolatile ferroelectric random access memories where bipolar polarization states are used to store the logic 0 and 1 data.Here the retention characteristics of BiFeO3 thin films with SrRuO3 bottom electrodes on both GdScO3 (110) and SrTiO3 (100) substrates are studied and compared,and the results of piezoresponse force microscopy provide a long time retention property of the films on two substrates.It is found that bismuth ferrite thin films grown on GdScO3 substrates show no preferred domain variants in comparison with the preferred downward polarization orientation toward bottom electrodes on SrTiO3 substrates.The retention test from a positive-up domain to a negative-down domain using a signal generator and an oscilloscope coincidentally shows bistable polarization states on the GdScO3 substrate over a measuring time of 500s,unlike the preferred domain orientation on SrTiO3,where more than 65％o of upward domains disappear after 1 s.In addition,different sizes of domains have been written and read by using the scanning tip of piezoresponse force microscopy,where the polarization can stabilize over one month.This study paves one route to improve the polarization retention property through the optimization of the lattice-mismatched stresses between films and substrates.