For example, I would like to display something like this:
It is myboldtext label.
But It is my's length cannot be expected, because, it may become 
Oh! It is my bold text label.
So, I can't use 3 labels to do so. Any recommends? Thanks.
For example, I would like to display something like this:
It is myboldtext label.
But It is my's length cannot be expected, because, it may become 
Oh! It is my bold text label.
So, I can't use 3 labels to do so. Any recommends? Thanks.
UILabel cannot do this out of the box.
I recommend using OHAttributedLabel, which is available on github.
You can still use 3 labels :)
Assuming the It is my label is label1, write a wrapper function that you always call to change label1's text; this function should also recalculate label1's size and reposition label2 and label3.  
-(void) setLabel1Text(NSString* newText)
{
    label1.text = newText;
    int label1TextLength = [label1.text sizeWithFont:label1.font];
    label2.x = label1.x + label1TextLength;
    label3.x = label2.x + label2.width;
}
And voila :)
 
    
    Using a 'UILabel', this is not strictly possible.  You can use bold fonts, however, if you want all text to be bold.  Reference thithis page for a list of fonts.  For example, instead of Helvetica, you can use Helvetica-Bold.  With plain UILabels, however, formatting is all or nothing; all the text has to be bold, or none of it is bold.  To display rich text with some parts bold, other parts not, however, check out NSMutableAttributedString.
Good luck! 
