A window curtain includes a rotary tube located in the top rail, and a curtain is connected between the top rail and the bottom rail. A scrolling device is located in the tube and includes a torsion spring mounted to a fixed shaft. A sleeve is connected to the fixed shaft. An adjustment unit is connected to the fixed shaft. The torsion spring is located between the sleeve and the adjustment unit. The adjustment unit includes a housing, a tubular part and a resilient member. The first end of the torsion spring is fixed to the tubular part. The torsion spring is compressed or released by rotating the adjustment unit. The tubular part includes ratchets which are engaged with a pawl in the housing to maintain the tubular part and the torsion spring from rotating reversely.