A window blind window blind includes a winding roller rollably mounted a window to lift or lower a roll-up piece, a blind curtain disposed forwardly of the roll-up piece and having elongate loop members, and a plurality of modular clip devices. Each loop member defines a lengthwise hole, and has an anchoring head portion having an access slot, marginal and gripped regions disposed at two sides of the lengthwise hole. Each clip device includes a grip body and a holding tongue. The grip body has loop-side and roll-up piece-side gaps to form a middle gripping finger portion, and internal and external gripping finger portions. The internal gripping finger portion and the holding tongue are placed in the lengthwise hole by virtue of inserting through the access slot to abut against the gripped and marginal regions, respectively. The roller-up piece is snugly engageable in the roll-up piece-side gap.