Thank you for your help @RuReady and @olyerickson. Now, I solved this problem.
I found out that there is working directly problem for this issue.
If I type getwd() in my RStudio console, then I got as below,
However, if I type getwd() in my RMarkdown as below, then I got as below,
The working directory is different in both output. That's why I was not able to load the figure. I don't know why it is showing different working directory. Maybe, the working directory in Rmarkdown is showing the where markdown file located ?
Thank you for your help!