ACAD SINICA,GRAD SCH,DEPT PHYS,BEIJING 100039,PEOPLES R CHINA
; NANKAI UNIV,DEPT PHYS,TIANJIN 300071,PEOPLES R CHINA
; ACAD SINICA,INST HIGH ENERGY PHYS,BEIJING 100039,PEOPLES R CHINA
Searching for a single top-quark production at LEP where the background is relatively clean is interesting. We study e(+)e(-) --> t ($) over bar b(($) over bar tb)+X cross sections at root s=200 GeV and root s=250 GeV energies in the framework of the minimal standard model and the extended standard model with two Higgs doublets. The resultant evaluation of the cross sections is not very encouraging, but can be optimistic and worth further serious studies in experiments and theories, if there exists charged Higgs with a large coupling parameter cot beta or the design luminosity is enhanced by several orders.