代碼如下：

UILabel *label = [[UILabel alloc] init];
label.text = labText; 

CGFloat width = label.frame.size.width;     //width : UILable的寬度
CGSize size = CGSizeMake(width, MAXFLOAT); 

CGFloat wordSize = 17;     //wordSize : 文字的大小
UIFont *font = [UIFont systemFontOfSize:wordSize];

//自動換行        
self.numberOfLines = 0;

 CGSize textRealSize;
 if (iOS7) { //iOS7以后的版本，包括iOS7
    textRealSize = [self.text boundingRectWithSize:size options:NSStringDrawingUsesLineFragmentOrigin     attributes:@{NSFontAttributeName:font} context:nil].size;
 } else { //iOS7以前的版本
    textRealSize = [self.text sizeWithFont:font constrainedToSize:size lineBreakMode:NSLineBreakByWordWrapping];
 }
 //改變高度
 CGRect rect = label.frame;
 rect.size.height = textRealSize.height;
 label.frame = rect;