We report the assisted role of water vapor in crystallographic cutting of graphene via iron catalysts in reduced atmosphere. Without water, graphene can be tailored with smooth trenches composed of straight lines with angles of 60℃ or 120℃ between two adjacent trenches. After the addition of water, new chacteristics are found: such as almost no iron particles can be detected along the trenches; each trench becomes longer and lots of graphene nanoribbons can be generated. The underlying mechanism is proposed and discussed, which is attributed to stimulating and lengthening of the catalytic activity of iron particles by water vapor. We report the assisted role of water vapor in crystallographic cutting of graphene via iron catalysts in reduced atmosphere. Without water, graphene can be tailored with smooth trenches composed of straight lines with angles of 60 ° C or 120 ° C between two adjacent trenches. After the addition of water, new chacteristics are found: such as almost no iron particles can be detected along the trenches; each trench becomes longer and lots of graphene nanoribbons can be generated. The underlying mechanism is proposed and discussed, which is attributed to stimulating and lengthening of the catalytic activity of iron particles by water vapor.