The effect of a catalyst on the growth and structure of carbon nanotube Y-junctions (CNTYs) using chemical vapor deposition has been investigated. Cobalt-, magnesium-, and calcium-nitrates are utilized as precursors of catalysts Co, Mg, Ca, Co/Mg, Co/Ca, and Mg/Ca for CNTY synthesis. Experimental result shows that Co/Mg or Co/Ca can grow CNTYs with straight branches while Co, Mg, Ca, and Mg/Ca will not grow any CNTYs, indicating that only combinations of Co with Mg or Ca will facilitate the formation of CNTYs. In addition, the effect of the carbon source on the formation of CNTYs has also been studied. It is found that thiophene (C4H4S) can promote the formation of CNTYs, while other sources such as methane (CH4) and acetylene (C2H2) cannot. The result shows that both the catalyst and the carbon source substantially affect the formation of CNTYs.