title_body: is there any software available to directly write in mobi format? is there any software in which I can directly write so that the book directly can be saved as .mobi file WYSIWYG format?
  
  I do not like to convert the book as it takes a lot of time and testing in kindle previewer. is there any solution maybe commercial or free or opensource?
upvoted_answer: Your question -- if I may restate -- is whether there are tools which allow you to create ebooks natively  for Kindle.
  
  Probably not, although it does have some specialized creation tools (Kindle Textbook Creator, > Kindle Kids' Book Creator, Kindle Comic Creator, Kindle Previewer). 
  
  My guess is that only Amazon.com could create such a tool because the .mobi standard is a proprietary format and is updated fairly regularly. 
  
  Some  tools have features  which let you export  to .mobi format (from MS word, PDF, or .mobi),  that includes Calibre  https://calibre-ebook.com/ Sigil, and Adobe InDesign. usually it converts from epub to mobi -- though calibre lets you convert from MS Word to mobi. But it isn't always a good conversion. 
  
  The thing is, aside from being a binary file, the Kindle format is trying to align closely to the epub format, so it makes sense (in my opinion) to focus on developing and testing an epub file -- and then after the fact converting the epub to mobi (and testing it and maybe  the styles so it works well on kindle devices and apps). 
  
  It's crazy and a waste of time to do all this; it would be a lot easier if Amazon.com  had adopted the epub standard. Certainly that would be better for publishers. But Amazon has come half-way and said if something is a valid EPUB file, then our Kindle Previewer app can reliably convert it to mobi. That's better than nothing. 
  
  For long term use, it's best to have an epub file -- because in the future it will be easier to convert from epub than to convert from .mobi