Variable expression for the same transgene construct has been documented in various plant species, regardless of the type of transgene. This phenomenon and the factors influencing it are reviewed. A variability in the transgene expression level was found to exist between and/or within independently derived lines, different lines of clonal replicates, siblings of the same line, a single plant, a single leaf as well as the same cell. Both the transgene dependent and the recipient dependent factors have been proposed to account for this phenomenon, the transgene dependent factors of which include transgene construct fidelity, T-DNA integration pattern, T-DNA copy number, promoter activity and the effect of nuclear matrix attachment region. In addition, different forms of epigenetic, homology-dependent gene silencing also contribute to the unstable expression of the identical transgene. The recipient dependent factors include position effect, ploidy level, genetic background, homozygosity, and developmental stage. Furthermore, environmental factors such as light intensity, temperature, field growth conditions and the season have been shown to modulate the levels of transgene expression. The approaches to stabilize the transgene expression were also discussed.