Abstract: Disclosed is an apparatus for ribbing cloth venetian blind strips in an efficient and mass production manner. The apparatus includes a group of strip pressing mechanisms which hold a strip to be ribbed in place and define a reference plane for other mechanisms in the apparatus during the ribbing operation. After the strip is pressed in place, a tubular space opening mechanism is actuated to open front ends of two previously flattened tubular spaces at two sides of the strip. Thereafter, two tubular space expanding mechanisms pushes the strip from the two sides thereof to vertically expand the already partially opened tubular spaces, allowing ribs to be fed into and extended through the expanded tubular spaces by a rib conveying mechanism. The cloth strips for venetian blind can therefore be accurate, quickly ribbed at two sides in a mass production manner.