An achromatic linear polarization rotator based on a tandem-2ϕ-twisted nematic liquid crystal cell (tandem-2ϕ-TNLC cell, where 2ϕ represents the total twisted angle) is theoretically analyzed and experimentally demonstrated. The tandem-2ϕ-TNLC cell comprises two conventional ϕ-TNLC cells with the required arrangement that the LC director close to the last layer of the first ϕ-TNLC cell should be perpendicular to that close to the first layer of the second ϕ-TNLC cell. With such a simple combination, the TNLC performances are considerably improved. According to the experimental results and theoretical analyses by Jones Calculus, the tandem-2ϕ-TNLC polarization rotator with suitable parameters is achromatic and insensitive to the polarization plane of incident light. Such properties provide these polarization rotators with potential for practical applications.