We first prove Sun's three conjectures [Z.H. Sun, On the number of incongruent residues of x(4) + ax(2) + bx modulo p, J. Number Theory 119 (2006) 210-241; Z.H. Sun, http://sfb.hytc.edu.cn/xsjl/szh/, 2000, June] on the number of rational points of some elliptic curves over finite fields F-p, which are related to the congruence cubic and quartic residue. And we provide some examples and comments concerning these conjectures. (c) 2007 Elsevier Inc. All rights reserved.