This paper describes "cell fibers" for engineering centimeter-scale 3D heterogeneous artificial tissues. Cells (both cell lines and primary cells) proliferate in our collagen/alginate core-shell hydrogel fibers, and completely fill the core space to form cell fibers. We found that the cell fibers keep their fibrous shapes even after removing the shell alginate hydrogel, and that they still maintain their bioactivities. As a demonstration, we constructed (i) tissue fabric sheets by weaving the cell fibers and (ii) 3D heterogeneous coil-like tissue tubes by reeling up two different cell fibers. These cell fibers would be an extremely effective tool as building elements for constructing large-scale functional tissues.