//定义一个可变字符串属性对象aStr

 NSMutableAttributedString *aStr = [[NSMutableAttributedString alloc]initWithString:str];

 //定义一个字典类对象dic存储设置的属性值，这里设置字体为粗体

 NSDictionary *dic = @{NSFontAttributeName:[UIFont boldSystemFontOfSize:self.myLabel.font.pointSize]   ,NSBackgroundColorAttributeName:[UIColor redColor]};

 //定义要改变的字符串的范围

     NSRange myRange = [str rangeOfString:title];

 //调用aStr对象的setAttributes方法设置属性

     [aStr setAttributes:dic range:myRange];

 //把aStr对象赋给Label

     self.myLabel.attributedText = aStr;