AbstractIt is necessary to increase the density of fibers of optical cables which are to be used in subscriber lines. To realize such high‐density fiber cables, multicore fibers with several cores inside a single cladding have been investigated. However, in multicore fibers, there is the problem of crosstalk between the adjacent cores.This paper investigates the two‐core asymmetric single‐mode fibers which reduce the crosstalk between adjacent cores. It is shown numerically that the crosstalk can be kept below a certain level for any fiber length, under the condition of fixed‐mode field diameter. In a symmetrical structure, the transfer of optical power is perfect and to suppress the crosstalk, the transmission length must be kept limited. However, in asymmetrical structures investigated in this paper, the transmission length is not limited and it becomes possible to design high‐density optical transmission lines with small separation between their