MgO　nanowires　were　synthesized　on　MgO(100)　and　Si(100)　substrates　by　MOCVD　with　β-diketonate　Mg(tmhd)2　(tmhd=2,2,6,6-tetramethyl-3,5-heptanedionate,　with　formula　C11H19O2).　Very　thin　gold　layers　(<5nm)　were　sputtered　onto　Si(100)　and　MgO(100)　substrates,　followed　by　heating　at　high　temperatures　to　form　catalyst　clusters.　The　alignment　of　MgO　nanowires　on　MgO(100)　substrates　are　mainly　along　[100]　but　they　are　in　[100]　and　[110]　alignments　on　Si(100)　substrate.　Whatever　the　substrates　are,　the　growth　direction　of　a　single　nanowire　is　normally　in　<001>　direction.　The　synthesis　temperature,　injection　period,　catalyst　starting　thickness,　oxygen　partial　pressure　and　injection　mass/period　have　strong　effects　on　the　synthesis　and　dimensions　of　nanowires.　It　shows　that　the　quantity　matching　between　the　two　reactive　species　(magnesium　and　oxygen)　is　very　important　for　the　vertical　growth　of　nanowires.　The　quantity　mismatching　between　the　reactive　species　is　subject　to　branching　structures　or　totally　kills　the　vertical　alignments　to　be　random　ones.　The　synthesized　MgO　nanowires　are　needle-shaped　with　a　decreasing　diameter　from　the　base　to　tip,　which　is　probably　due　to　the　lateral　growth　as　well　as　the　catalyst　diffusion　during　the　synthesis.　©　2011　IEEE.